How to Diagnose and Fix Everything Electronic

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					How to Diagnose
and Fix Everything
Electronic
Michael Jay Geier




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This book is dedicated to my parents, for putting up with their young son’s taking
everything in the house apart, even though it scared them to death; to my brother, for
providing me with a steady stream of broken items to fix and the encouragement to figure
them out; to Greg, for sharing countless happy teenage hours fiddling with circuits, projects
and walkie-talkies; to Rick, for always believing in and promoting my talents; to Cousin
Jerry, for some of my earliest guidance in electronic exploration; and to Alvin Fernald and
Tom Swift Jr., whose fictional technological exploits kept me spellbound through most of my
childhood and made me believe anything was possible with a handful of transistors and the
know-how to make them wake up and do something.
About the Author
Michael Jay Geier has been an electronics technician, designer and inventor since age 6.
He took apart everything he could get his hands on, and soon discovered that learning
to put it back together was even more fun. By age 8, he operated a neighborhood
electronics repair service that was profiled in The Miami News. He went on to work
in numerous service centers in Miami, Boston and Seattle, frequently serving as the
“tough dog” tech who solved the cases other techs couldn’t. At the same time, Michael
was a pioneer in the field of augmentative communications systems, helping a noted
Boston clinic develop computer speech systems for children with cerebral palsy. He
also invented and sold an amateur radio device while writing and marketing software
in the early years of personal computing.
     Michael holds an FCC Extra-class amateur radio license. His involvement in ham
radio led to his writing career, first with articles for ham radio magazines, and then with
general technology features in Electronic Engineering Times, Desktop Engineering, IEEE
Spectrum, and The Envisioneering Newsletter. His work on digital rights management
has been cited in several patents. Michael has a Boston Conservatory of Music degree
in composition, was trained as a conductor, and is an accomplished classical, jazz and
pop pianist, and a published songwriter. Along with building and repairing electronic
circuitry, he enjoys table tennis, restoring old mopeds, ice skating, bicycling, and
banging out a jazz tune on the harpsichord in his kitchen.
   Contents
              Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
              Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
              Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

Chapter 1       prepare for Blastoff: Fixing Is Fun!                          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .   1
    Repair: Why Do It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                      2
    Is It Always Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        3

Chapter 2       Setting Up Shop: tools of the trade  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .                            5
    Must Haves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     5
         A Good Place to Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                              5
         Digital Multimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          8
         Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        9
         Analog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    9
         Digital . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  10
         Analog with Cursor Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       12
         Analog with Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                            12
         PC-Based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     13
         Buying an Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                             13
         Soldering Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       14
         Plastic-Melting Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                         15
         Solder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   15
         Desoldering Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          16
         Hand Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       18
         Magnifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    19
         Clip Leads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     19
         Swabs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  20
         Contact Cleaner Spray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                              20
         Alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    20
         Naphtha . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    20
         Heatsink Grease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          21
         Heat-Shrink Tubing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                           21
         Electrical Tape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                      21


                                                                                                                                              v
vi    Contents
                 Small Cups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         21
                 Internet Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          22
            Nice-to-Haves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     22
                 Digital Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           22
                 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           22
                 Transistor Tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          23
                 Capacitance Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              23
                 Signal Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           23
                 Frequency Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              24
                 Analog Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         24
                 Isolation Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              25
                 Stereo Microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            25
                 Bench Vise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         26
                 Hot-Melt Glue Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              26
                 Magnet on a Stick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            26
                 Cyanoacrylate Glue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             26
                 Component Cooler Spray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   27
                 Data Books . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         27
                 Parts Assortment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           27
                 Scrap Boards for Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             28
            Wish List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   29
                 Inductance Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             29
                 Logic Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           29
                 SMT Rework Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               29
                 Spectrum Analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              30

     Chapter 3 Danger, Danger! Staying Safe  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 31
            Electric Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      31
            Physical Injury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     33
            Your Turn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   34
                 Electrical Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            34
                 Physical Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            36
            You Fixed It! Is It Safe? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         38

     Chapter 4 I Fix, therefore I am: the philosophy of troubleshooting  .  .  .  .  .  . 39
            Why Things Work in the First Place . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    40
            Products as Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     40
            If It Only Had a Brain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          41
            The Good, the Bad and the Sloppy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    42
            Mistakes Beginners Make . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               42
                   Adjusting to Cover the Real Trouble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    42
                   Making the Data Fit the Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   43
                   Going Around in Circles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              43
            That’s How It Goes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          44
                   Infant Mortality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       44
                   Mechanical Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          45
                   Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        45
                                                                                                            Contents            vii
          Solder Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       46
          Heat Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       47
          Electrical Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       47
          Physical Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       49
          The Great Capacitor Scandal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   50
    History Lessons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       51
    Stick Out Your USB Port and Say “Ahhh”: Initial Evaluation . . . . . . . . . . . . . . . . .                                52
          Use Your Noodle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           54

Chapter 5       Naming Names: Important terms,
                 Concepts and Building Blocks  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 59
    Electrical Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         59
    Circuit Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        61
    Signal Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       63
    Building Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       65

Chapter 6       Working Your Weapons: Using test equipment  .  .  .  .  .  .  .  .  .  .  .  .  .  . 71
    Digital Multimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         71
          Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       71
          DC Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       72
          AC Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       73
          Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       73
          Continuity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       74
          DC Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         75
          Diode Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       76
    Oscilloscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     76
          Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       77
          Screen Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          78
          Vertical Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        78
          Horizontal Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            78
          Trigger Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         79
          Viewing a Real Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              79
          What All Those Knobs Do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  79
          Digital Differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            96
    Soldering Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       98
    Desoldering Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         101
          Wick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    101
          Suckers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     102
          Rework Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         102
    Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      103
          Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        103
          Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   105
          Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     105
    Transistor Tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     105
    Capacitance Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         106
    Signal Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        107
    Frequency Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           107
viii   Contents
         Analog Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
         Contact Cleaner Spray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
         Component Cooler Spray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

   Chapter 7         What Little Gizmos are Made of: Components  .  .  .  .  .  .  .  .  .  .  .  .  . 113
         Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      113
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        113
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    115
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             115
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              117
         Crystals and Resonators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             117
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        118
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    118
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             118
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              118
         Crystal Clock Oscillators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             118
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        119
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    119
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             120
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              120
         Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    120
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        121
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    121
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             121
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              122
         Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   122
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        122
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    123
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             123
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              124
         Inductors and Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  124
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        125
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    125
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             125
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              125
         Integrated Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         126
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        128
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    128
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             128
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              128
         Op Amps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       128
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        129
               Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    129
               What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             130
               Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              130
         Resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   130
               Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        131
                                                                                                                Contents           ix
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      132
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               132
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                133
    Potentiometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         133
         Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          134
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      134
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               135
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                135
    Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   135
         Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          136
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      136
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               137
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                137
    Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     137
         Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          138
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      138
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               139
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                139
    Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      139
         Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          141
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      142
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               142
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                142
    Voltage Regulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           143
         Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          143
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      144
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               144
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                144
    Zener Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         145
         Markings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          145
         Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      145
         What Kills Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               146
         Out-of-Circuit Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                146

Chapter 8       roadmaps and Street Signs: Diagrams  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 147
    Hooked on Tronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            150
         Call Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              152
    The Good, the Not Bad, and the Miserable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                           153
         The Good . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          153
         The Not Bad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             153
         The Miserable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             153
    Once Upon a Time… . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                154
         Amplifier Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             154
         Switching Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    159
         Push-Pull Audio Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                     161
         Mega Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             163
x   Contents
          Give It a Try . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      164
                Radios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       164
                CD and DVD Players . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 164
                Keep Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             166
          But I Ain’t Got One! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           166
          Your Wish Is Not My Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        166

    Chapter 9 entering Without Breaking: Getting Inside  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 171
          Separating Snaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           173
          Removing Ribbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             174
          Pulling Wire Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                175
          Layers and Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            175
          Opening a Shut Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              177
                Receivers and Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   177
                VCRs, CD and DVD Players . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       177
                TVs and LCD Monitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                   177
                Turntables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         178
                Video Projectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             178
                Portable DVD Players with LCD Screens . . . . . . . . . . . . . . . . . . . . . . . . . . .                              179
                MP3 Players . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          180
                PDAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       181
                Cell Phones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .          181
                Camcorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           181
                Digital Cameras . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            182
                Laptop Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               183

    Chapter 10          What the heck Is that? recognizing Major Features  .  .  .  .  .  .  .  . 187
          Power to the Circuit: Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       187
          Follow the Copper-Lined Road: Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                        191
          Shake, Bake, Slice and Dice: Signal Processing . . . . . . . . . . . . . . . . . . . . . . . . . . .                           192
          Out You Go: Output Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                  194
          A Moving Tale: Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    195
          Danger Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        198

    Chapter 11          a-hunting We Will Go: Signal tracing and Diagnosis                                           .  .  .  .  .  .  . 199
          Dead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   199
          Comatose or Crazy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            202
          Alive and Awake but Not Quite Kicking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          202
          Sometimes Yes, Sometimes No . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                      203
          To and Fro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       205
          All the World’s a Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            206
          Check, Please . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .        207
          When All Else Fails: Desperate Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                          208
                Shotgunning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            208
                Current Blasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             209
                LAP Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .             210
                                                                                                               Contents           xi

Chapter 12        presto Change-O: Circuit Boards and replacing Components  .  .  . 211
    Through-Hole Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            211
    Surface-Mount Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    213
    Choosing Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               214
          Ye Olde Junque Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                214
          Parts Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            214
          Substitutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       215
    Installing the New Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            220
          Through-Hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            221
          SMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     222
    Finding Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       223
    Saving Damaged Boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               223
    LSI and Other Dirty Words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 224

Chapter 13        that’s a Wrap: reverse-Order reassembly  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 227
    Common Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           227
    Getting Started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .       228
    Reconnecting Ribbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .              229
    Oops! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   230
    Layers and Cups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           231
    Oh, Snap! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .     231
    Screwing It Up Without Screwing It Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                       232
    Done! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   232

Chapter 14        aces Up Your Sleeve: tips and tricks for Specific products  .  .  . 233
    Switching Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                233
          How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               233
          What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 234
          Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         234
          The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                234
          How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            235
    Audio Amplifiers and Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                    236
          How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               236
          What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 237
          Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         238
          The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                238
          How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            238
    Disc Players and Recorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                240
          How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               240
          What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 241
          Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .         242
          The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                242
          How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .            243
    Flat-Panel Displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .           248
          How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .               248
          What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                 249
xii      Contents
                    Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    249
                    The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           249
                    How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       250
               Hard Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                 251
                    How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                          251
                    What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                            251
                    Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    252
                    The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           252
                    How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       252
               Laptop Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                      253
                    How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                          253
                    What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                            253
                    Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    255
                    The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           256
                    How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       256
               MP3 Players . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                 261
                    How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                          261
                    What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                            261
                    Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    262
                    The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           262
                    How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       262
               VCRs and Camcorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                         262
                    How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                          263
                    What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                            265
                    Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    265
                    The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           266
                    How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       266
               Video Projectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                  277
                    How They Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                          277
                    What Can Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                            278
                    Is It Worth It? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                    279
                    The Dangers Within . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                           279
                    How to Fix One . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                       280
               Have at It! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                               284

      GLOSSarY  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 285
               Common Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

      Index           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . 301
Foreword
There is a keen sense of personal accomplishment to be gained from fixing something
yourself that might otherwise have cost a fortune to repair or been recycled ahead of
its time. Michael Jay Geier has known this joy since childhood. Now, for the first time,
he shares his secrets in written and pictorial form.
      I’ve known Michael across three decades, multiple time zones and dozens
of entertainment and technology projects and consulting gigs we’ve tackled
together. Quite simply, Michael sees electronic products as songs or symphonies
of components, specialized parts working in harmony when they leave the
manufacturer.
      Yet individual musicians may be missing or off-key when the product fails from
age, misuse or random component failure. Like the keen orchestra conductor he
was trained to be, Michael quickly zeroes in on what parts of a broken electronic
product are out of tune, using many skills he will teach you in this book, along with
instruments to sense and measure things beyond the human senses of sight and
sound. Fixing and extending the life of products we love, including things no longer
being made and for which there is no ready replacement, is a valuable skill worth
developing and nurturing.
      Musicians see patterns well and communicate their art to larger audiences.
Michael’s expertise in troubleshooting consumer electronics is unmatched. Here, he
shares the patterns that come easily to him with a broad audience of readers who
want to enjoy their consumer electronics products longer, can’t afford traditional
repairs, fear their favorite irreplaceable gadget could be lost or further damaged while
at the shop, or want to keep alive something old or obscure enough that no repair
facility has the resources to work on it.
      I put myself through college repairing consumer electronics and entertainment
products. As much as I learned, when I first met Michael, I knew within days that he
had a gift for troubleshooting far faster than my own. Jealousy soon faded as I saw he
was confident and professional in his communications skills, and he loved sharing his
insights and tricks.
      Many TV, camcorder and video player manufacturers have employed lessons
learned from Michael’s shop repairs to improve their next-generation products and
make them more durable and dependable. Michael has made his mark on improving
product designs for more than two decades now.

                                                                                   xiii
xiv   Foreword
           I personally know several consumer electronics repair shop employers who
       hated to see Michael go. Yet not a one denied that Michael’s time there and sharing
       of expertise made the entire shop better at repair and gave customers their serviced
       products back faster and with more reliability against ever failing again.
           Herein, Michael shares his hunches, skills and insights at a level any dedicated
       reader can absorb and apply. Enjoy the satisfaction that comes with learning to repair
       your own equipment. And spread the word—it’s about as green and economically
       smart as you can be!
                                                           Richard Doherty, Research Director
                                                                    The Envisioneering Group
                                                                             Seaford, New York
                                                                                      July 2010
Acknowledgments
To Neil Salkind, Roger Stewart, Joya Anthony and the other wonderful folks at
McGraw-Hill, many thanks for recognizing the value of this material and shepherding
it into existence. It takes a team to raise an idea. You’ve been a great team, and I’m
honored to have been part of it.




Introduction
Everything. That’s a scary word, one I almost avoided including in the title. Can any
book actually cover everything about a topic? Yes and no. Yes, in the sense that the
principles and techniques you’ll learn can be applied to the repair of every kind of
consumer electronics device presently being made or likely to be sold in the near
future. No, in that it’s impossible to fit each of the thousands of types of components
and countless varieties of gadgets in the world into one volume. Covering all of them
in deepest detail would take a library, and a good-sized one at that.
     The focus of this book is on today’s electronics, most of which are digital in
nature, and the kinds of problems you’re most likely to encounter. It might seem
like there isn’t that much one can service in modern digital gear, compared to the
older analog circuitry. Dense boards populated by rows of chips with leads too close
together even to poke at with a test probe don’t seem like good repair candidates, do
they? Luckily, those areas aren’t where most failures occur, and there’s still plenty of
accessible circuitry to work on! In fact, some common problems in today’s gear were
rare or nonexistent in earlier technology, and they’re quite reparable.
     Exotic and very obsolete components and their associated products aren’t covered
in this book. Electron tubes, once the mainstay of all electronics, are pretty much
gone, so we won’t spend time on their peculiarities and specific troubleshooting
methods. If you want to repair tube-type guitar amplifiers, you can find books
dedicated to them. Similarly, we won’t be discussing microwave ovens, which also
have tubes, or transmitting amplifiers of the sort used by amateur radio operators. Nor
will we take more than a passing glance at cathode ray tube (CRT, or picture tube)–
based TVs and monitors. The CRT had a good long run, from the 1940s until just a few
years ago, but it’s a dead technology, thoroughly supplanted by flat-panel displays.
Servicing CRT sets is rather dangerous, so please find a book devoted to them if you
have an interest in, say, restoring antique TVs. What’s covered here is relevant but
not comprehensive enough regarding that topic to keep you safe around those high-
voltage beasts.

                                                                                     xv
xvi   Introduction
            Some obsolete technology is still in common use and may remain so for years to
       come, so we’ll explore it. Tape-based video recording continues to be used in some
       digital camcorders. VCRs, which are rapidly disappearing as high-definition TV
       (HDTV) obsoletes them, may be the only key to recovery of precious home movies
       yet to be transferred to digital media. Serious audio devotees treasure their analog
       tape recorders and turntables and will never replace them with CD or MP3 players.
       We won’t spend much time on the old formats, but the troubleshooting techniques
       covered here are applicable to their repair.
            Most of today’s digital equipment still contains analog circuitry for audio or video
       output, microphone input, voltage regulation and such. Home theater receivers
       use analog amplifier stages, and many have old-fashioned, linear power supplies
       as well, because they’re electrically quieter than newer, pulse-driven designs. In
       fact, the best audiophile-grade stereo gear is pretty much all analog and will likely
       remain that way. Even digital radio and TV receivers use analog stages to amplify and
       separate incoming signals before digital decoders extract the data. So, troubleshooting
       techniques specific to analog circuitry are far from antiquated; they continue to be
       relevant in our digital era.
            In this book, it is assumed that you have probably opened an electronic device
       at one time or another and checked a fuse. Perhaps you know a resistor when you
       see one, and maybe you’ve even soldered or done some basic troubleshooting. Still,
       we’re going to start from the top, ensuring you’re a sound swimmer before diving into
       the deep end. And dive we will! Beginning with a look at the tools you’ll need, we’ll
       explore setting up your home workshop. We’ll discuss the best types of workbenches
       and lamps, and where to put your gear and tools. We’ll take a close look at the most
       useful test instruments, where to find bargains on them, and how to operate them.
       Getting good with an oscilloscope is key to being a crack shot tech, so we’ll explore a
       scope’s operation in detail, button by button.
            Using other test equipment like digital voltmeters and ohmmeters is also crucial
       to effective repair. We’ll focus on commonly available test gear, without spending
       significant effort on very expensive, exotic instruments you’re never likely to own.
       We’ll examine how to take a product apart, figure out what’s wrong with it, replace
       parts and close it back up again. Finally, we’ll look at tips and tricks for specific
       devices, from optical disc players to video recorders and receivers. Here’s a quick
       breakdown of what’s in each chapter.

       Chapter 1, “Prepare for Blastoff: Fixing Is Fun!”
        •	 Why repair things? Environmental and economic factors, learning, fun, preserving
           rare and obsolete technology, potential profit.
        •	 When is a product worth repairing, and when is it better to cut it up for parts?

       Chapter 2, “Setting Up Shop: Tools of the Trade”
        •	 Necessary items, from hand tools to test instruments, and how to buy them. Must-
           haves, nice-to-haves, and expensive goodies to dream about.
        •	 How to select a workbench and set it up, and where to put it.
                                                              Introduction     xvii

Chapter 3, “Danger, Danger! Staying Safe”
•	 How to avoid getting hurt while servicing electronics: electrical and physical
   hazards, eye and ear protection.
•	 How not to damage the device you’re repairing: causing electrical and physical
   damage.
•	 Ensuring user safety after product repair.

Chapter 4, “I Fix, Therefore I Am:
The Philosophy of Troubleshooting”
•	 General troubleshooting principles: why things work, why they stop.
•	 Common mistakes and how to avoid them.
•	 Organization of modern devices: microprocessor brains, nervous system, muscles
   and senses.
•	 The “art” side of electronics: manufacturer-specific quirks and issues.
•	 What fails most often and why.
•	 Failure history and how it helps diagnose problems.
•	 Preliminary diagnosis based on symptom analysis: dead, comatose and nearly
   working.
•	 Case histories.

Chapter 5, “Naming Names: Important Terms,
Concepts and Building Blocks”
•	 Electrical units: volts, amps, resistance, capacitance, and so on.
•	 Circuit concepts: how parts connect and how current moves through them. Series
   and parallel.
•	 Signal concepts: how changes in voltage represent information. Waveforms.
   Analog and digital representation.
•	 Building blocks: common circuits used in many products. Amplifiers, oscillators,
   frequency synthesizers and power supplies.

Chapter 6, “Working Your Weapons: Using Test Equipment”
•	 Digital multimeter: measuring voltage, current and resistance.
•	 Oscilloscope: detailed, button-by-button operation, including delayed sweep
   measurements.
•	 AC and DC signal components, rolloff and other issues affecting measurement
   strategy.
•	 Soldering and desoldering techniques.
•	 Bench power supply: voltage and current considerations, DC plug polarity.
•	 Transistor tester.
•	 Capacitance meter.
•	 Signal generator.
xviii   Introduction
         •	 Frequency counter.
         •	 Analog meter: when to use it, interpreting the wiggling meter needle, tests not
            possible with a digital instrument.
         •	 Contact cleaner spray: what to use it on, what not to.
         •	 Component cooler spray: solving thermal intermittents, considerations for safe use.

        Chapter 7, “What Little Gizmos Are Made of: Components”
         •	 Common parts: capacitors, clock oscillators, crystals, diodes, fuses, inductors
            and transformers, integrated circuits, op-amps, resistors, potentiometers, relays,
            switches, transistors, voltage regulators and zeners.
         •	 Varieties of each type of part.
         •	 Symbols, markings and photos.
         •	 Uses: what components do in circuits.
         •	 What kills them.
         •	 How to test them out of circuit.

        Chapter 8, “Roadmaps and Street Signs: Diagrams”
         •	   Block, schematic and pictorial diagrams.
         •	   Learning to read diagrams like a story: signal flow, organization in stages.
         •	   Symbols and call numbers.
         •	   Good, average and bad diagrams.
         •	   Part-by-part analysis of individual stages and their functions. Amplifier example.
         •	   Organization of larger structures. Switching power supply example.
         •	   Practicing reading: looking for stages and structures in radios and DVD players.
         •	   Working without a diagram.
         •	   Case history of troubleshooting an LCD TV without a schematic.

        Chapter 9, “Entering Without Breaking: Getting Inside”
         •	   Separating case halves: hidden snaps.
         •	   Disconnecting ribbon cables.
         •	   Layers: disassembling in order, use of digital photos and nested cups.
         •	   Disassembly tips for common products: receivers, VCRs, DVD players, flat-panel
              TVs, turntables, video projectors, MP3 players, PDAs, cell phones, camcorders,
              digital cameras and laptop computers.

        Chapter 10, “What the Heck Is That:
        Recognizing Major Features”
         •	 What various sections of circuitry look like: descriptions and photos.
         •	 Recognizing sections from components specific to their functions: inductors,
            power transistors, and so on.
         •	 Power supplies: linear and switching.
         •	 Backlight inverters.
                                                                Introduction     xix
•	 Signal processing areas, analog and digital.
•	 Digital control sections.
•	 Output stages: discrete transistors and integrated modules.
•	 Mechanisms: video head drum, capstan motor, laser optical head and DLP
   color wheel.
•	 Danger points.

Chapter 11, “A-Hunting We Will Go:
Signal Tracing and Diagnosis”
•	 Where to begin, based on observed symptoms.
•	 Dead, comatose or crazy, alive and awake but not quite kicking.
•	 Intermittents: thermal and mechanical, bad solder joints, board cracks, positional
   and vibration-sensitive.
•	 Working forward or backward through stages: when each technique is appropriate.
•	 Stages, test points and making sure you’re in the right place.
•	 Zeroing in on bad components.
•	 Desperate measures: shotgunning, current blasting and LAP method.

Chapter 12, “Presto Change-O: Circuit Boards
and Replacing Components”
•	 Desoldering through-hole and surface-mount components.
•	 Choosing replacement parts: new, from your stash and from parts machines.
•	 Substituting similar parts when you can’t get the exact replacement: vital
   characteristics that must be matched or exceeded, and allowable differences in
   capacitors, diodes, resistors, transistors and zeners.
•	 Installing new parts: through-hole and surface-mount, mounting power
   transistors.
•	 Finding components: standardized, proprietary, local, mail-order, new and
   surplus.
•	 Saving damaged boards: bridging broken conductors and bad layer interconnects.
•	 Reflowing solder on high-density integrated circuit chips.

Chapter 13, “That’s a Wrap: Reverse-Order Reassembly”
•	 Common reassembly errors.
•	 Ensuring good ground connections on boards and chassis.
•	 Lead dress: placement of wires and cables, physical and thermal risks,
   electromagnetic interference.
•	 Reconnecting ribbon cables.
•	 Repairing damaged ribbon sockets.
•	 Reversing layer and cup order.
•	 Rejoining plastic snaps.
•	 Reinserting screws: tension and correct placement.
•	 Final test.
xx   Introduction

       Chapter 14, “Aces Up Your Sleeve:
       Tips and Tricks for Specific Products”
        •	 How they work, what can go wrong, when repair is worth doing, dangers within,
           and how to fix them.
        •	 Switching power supplies, receivers, disc players and recorders, flat-panel displays,
           hard drives, laptop computers, MP3 players, VCRs, camcorders and video projectors.

            Whether or not you’ve already had your hands inside some electronic devices,
       this book will guide you from the “maybe it’s the fuse” level to the “ah, the biasing
       diode on the output stage is open” point. It will help hone your sleuthing skills with
       logic and a solid foundation in how things work, until you feel like an ace detective
       of electrons. At the very least, it’ll leave you fascinated with everything that goes
       on inside your favorite gadgets and eager to tackle everything that comes your way.
       Everything…maybe it’s not such a scary word after all.
Chapter                  1
 Prepare for Blastoff:
 Fixing Is Fun!

 E    lectronics is a lifelong love affair. Once its mysteries and thrills get in your blood,
      they never leave you. I became fascinated with circuits and gadgets when I was
 about 5 years old, not long after I started playing the piano. There may have been
 something of a connection between the two interests—both involved inanimate objects
 springing to life by the guidance of my mind and hands. Building and repairing radios,
 amplifiers and record players always felt a little like playing God, or perhaps
 Dr. Frankenstein: “Live, I command thee!” A yank on the switch, just like in the movies,
 and, if I had figured out the puzzle correctly (which was far from certain at that age),
 live it would! Pilot lights would glow, speakers would crackle with music and faraway
 voices, and motors would turn, spinning records that filled my room with Haydn,
 Berlioz and The Beatles. It was quite a power trip (okay, a little pun intended) for a kid
 and kept me hankering for more such adventures.
      By age 8, I was running my own neighborhood fix-it business, documented in
 an article by The Miami News titled “Little Engineer Keeps Plugging Toward Goal.”
 Repairs usually ran about 25 cents, and I had customers! Neighborhood pals, their
 families and my dad’s insurance business clients kept me busy with malfunctioning
 radios and tape recorders. I even fixed my pediatrician’s hearing tester for 50 cents.
 If only I’d known what he was charging….
      My progression from such intuitive tinkering to the understanding required for
 serious technician work at the employable level involved many years of hands-on
 learning, poking around and deducing which components did what, and tracing signals
 through radio stages by touching solder joints with a screwdriver while listening for the
 crackling it caused in the speaker. Later came meters, signal tracers and, finally, the
 eye-opening magic window of the oscilloscope.
      Ah, how I treasure all the hours spent building useful devices like intercoms
 and fanciful ones like the Electroquadrostatic Litholator (don’t ask), fixing every
 broken gadget I could get my hands on, and devouring Popular Electronics, Electronics
 Illustrated and Radio-Electronics—great magazines crammed with construction articles
 and repair advice columns. Only one issue a month? What were they waiting for??

                                                                                           1
2   How to Diagnose and Fix Everything Electronic

    C’mon, guys, I just have to see the last part of that series on building your own color
    TV camera, even though I’ll never attempt it. But now I know how a vidicon tube
    works! And, thanks to my parents’ wise and strict rule that I experiment only on
    battery-powered items, I survived my early years to share my enthusiastically earned
    expertise with you, the budding tech.
         After graduating from the Boston Conservatory of Music, I did what any highly
    trained, newly certified composer/conductor does: I completely abandoned my
    field of study and started working in electronics! I was a tech in repair shops, I
    programmed computers, and I developed circuitry and software for several companies
    around Boston and New York, while building my own inventions and running a
    little mail-order company to sell them. All of those experiences integrated into
    the approach I will present in this book, which includes inductive and deductive
    reasoning, concepts of signal flow and device organization, taking measurements,
    practical skills and tips for successful repair, a little bit of art, and even a touch of
    whimsy here and there.
         No book can make you an expert at anything; that takes years of experience and
    squirreling away countless nuggets of wisdom gleaned from what did and didn’t work
    for you. My hope is that this distillation of my own hard-won understanding will
    infect you with the love of circuits and their sometimes odd behaviors, and start you
    on the very enjoyable path of developing your skills at the wonderful, wacky world of
    electronic repair.
         So, warm up your soldering iron, wrap your fingers around the knobs of that
    oscilloscope and crank up the sweep rate, 'cause here we go!



Repair: Why Do It?
    When I was a kid, there were radio and TV servicers in many neighborhoods. If
    something broke, you dropped it off at your local electronics repair shop, which was
    as much a part of ordinary life as the corner automotive service garage. These days,
    those shops have all but disappeared as rising labor costs and device complexity have
    driven consumer electronics into the age of the disposable machine. When it stops
    working, you toss it out and get a new one. So why fix something yourself? Isn’t it
    cheaper and easier just to go out to your local discount store and plunk down the ol’
    credit card?
        It might be easier, but it’s usually not cheaper! Sans the cost of labor, repair can
    be quite cost effective. There are lots of other good reasons to become a proficient
    technician, too:

     •	 It’s fun. You’ll get a strong sense of satisfaction when your efforts yield a properly
        working gadget. It feels a bit like you’re a detective solving a murder case, and it’s
        more fun to use your noodle than your wallet.
     •	 It’s absorbing. Learning to repair things is a great hobby to which you can devote
        many fruitful hours. It’s good for your brain, and it beats watching TV any day
        (unless you fixed that TV yourself!).
                            Chapter 1       Prepare for Blastoff: Fixing Is Fun!            3

    •	 It’s economical. Why pay retail for new electronics when you can get great
       stuff cheap or even free? Especially if you live in or near a city, resources like
       craigslist.org will provide all the tech toys you want, often for nothing. Lots of
       broken gadgets are given away, since bringing them in for repair costs so much.
       They’re yours for the taking. All you have to do is fix ’em!
    •	 It can be profitable. Some of the broken items people nonchalantly discard are
       surprisingly valuable. When your tech skills become well developed, you’ll be able
       to repair a wide variety of devices and sell what you don’t want for yourself.
    •	 It can preserve rare or obsolete technology. Obsolete isn’t always a negative term!
       Some older technologies were quite nice and have not been replaced by newer
       devices offering the same features, utility or quality. The continued zeal of analog
       audio devotees painstakingly tweaking their turntables offers a prime example of
       the enduring value of a technology no longer widely available.
    •	 It’s green. Every product kept out of the landfill is worth two in ecological terms:
       the one that doesn’t get thrown away and the one that isn’t purchased to replace
       it. The wastefulness of tossing out, say, a video projector with a single capacitor
       is staggering. To rip off an old song, “Nothing saves the green’ry like repairing the
       machin’ry in the morning….”
    •	 Your friends and family will drive you crazy. Being a good tech is like being a doctor:
       everyone will come to you for advice and help. Okay, maybe this one isn’t such
       an incentive, but it feels great to be able to help your friends and loved ones,
       doesn’t it? Being admired as an expert isn’t such a terrible thing either.



Is It Always Worth It?
   While it’s often sensible to repair malfunctioning machines, sometimes the endeavor
   can be a big waste of time and effort, either because the device is so damaged that
   any repair attempt will be futile or the cost or time required is overwhelming. Part
   of a technician’s expertise, like a doctor’s, lies in recognizing when the patient can
   be saved and when it’s time for last rites and pulling the plug—in this case, literally!
   Luckily, in our silicon and copper realm, those destined for the hereafter can be
   recycled as parts. A stack of old circuit boards loaded with capacitors, transistors,
   connectors and other components is as essential as your soldering iron, and you’ll
   amass a collection before you know it.
This page intentionally left blank
Chapter                    2
   Setting Up Shop:
   Tools of the Trade

   T    o repair anything, you will need some basic test gear and a suitable place to use
        it. Because electrons and their energy flows are invisible, test equipment has
   been around almost as long as human awareness of electricity itself. The right test
   instruments and hand tools enable you to get inside a product without damaging it,
   find the trouble, change the bad parts and reassemble the case correctly and safely.



Must Haves
   Electronics work can involve a seemingly unending array of instruments, but you don’t
   need them all. Some of them are insanely expensive and only rarely useful. Others
   cost a lot less and find application in almost every circumstance. Some items are
   absolutely essential, so let’s start by looking at those things you can’t live without, and
   how and where to set them up for the most effective, efficient service environment.


   A Good Place to Work
   Like surgery, tech work is exacting; there’s little room for error. One slip of the test
   probe can cause a momentary short that does damage worse than the problem you
   were trying to solve. One of the most important elements of effective, conscientious
   repairing is an appropriate workspace set up to make the task as easy and comfortable
   as possible, minimizing the likelihood of catastrophic error.
       First, consider your location. If you have young children, it’s imperative that
   the workbench is set up in a room that can be locked. Opened electronic products
   and the equipment used to service them are not child-safe, and the last thing you or
   your kids need is an accident that could injure them. Dens and basements can be




                                                                                            5
6   How to Diagnose and Fix Everything Electronic

    suitable locations, but garages are probably best avoided if the kids are still at that
    “poke in a finger and see what happens” age. Pets, too, can wreak all kinds of havoc
    on disassembled machinery. Cats love to climb on and play with things, particularly
    if those things are warm. The effects can range from lost screws and broken parts to
    dead cats! Let’s face it, cats are not big readers, and a “Danger! High Voltage!” sticker
    looks about the same to them as “Cat Toy Inside.” Keep kitty away from your repair
    work, even when you’re in the room. You just never know when the little angel sitting
    there so placidly will make a sudden leap at your project and turn it into op-art.
         Many of us have our workshops in the basement. This location is a mixed bag.
    It keeps the somewhat messy business of repair out of your living space, but it has
    some drawbacks. If you live in a cooler clime, it can get mighty chilly down there in
    the wintertime! Worse, basements tend to be damp, which is bad for your test gear.
    In damp environments, oscilloscopes and meters have a way of not working if you
    haven’t used them for awhile, because moisture gets into connectors and redirects
    normal current paths in unpredictable ways. Still, the basement may be your best bet.
    Just be sure to fire up your gear now and then to dry it out, and run a dehumidifier if
    humidity climbs above 70 percent or so. Use an electric heater in the winter; kerosene
    heaters designed for indoor operation still emit quite a bit of carbon dioxide that will
    build up in the unventilated spaces of most basements. And should such a heater
    malfunction and put out a little carbon monoxide, you’ll probably be dead before
    becoming aware of anything wrong.
         The workbench itself should be as large as you can manage, with plenty of space
    for your test equipment, soldering iron, power supplies and other ancillary gear along
    the edges; you’ll need to keep the center clear for the item to be repaired. Wonderful,
    prefab test benches can be mail-ordered, but they’re fairly expensive and are most
    often found in professional shops. If you have the means, go for it. Get one with
    shelves and lots of power strips. If, like most of us, you’d rather not spend hundreds
    of dollars on a bench, there are plenty of alternatives. You can make your own in the
    time-honored way, from an old solid door (hollow doors aren’t strong enough) and
    some homemade wooden legs and braces. If you’re not the woodworking type, a big
    desk can sometimes suffice.
         Sturdy desks and tables suitable as workbenches can often be had for very little
    from thrift stores or for free from online trading boards, because of one factor in your
    favor: they don’t have to be pretty. In fact, avoid spotless, fancy furniture, because
    you’ll feel bad when you nick, scrape, singe and accidentally drill holes in it. An Ikea-
    style desk works great, as long as it’s well-braced and sturdy. A white Formica surface
    is nice too, because you can see dropped screws and such much more easily than
    with a darker, textured covering (see Figure 2-1). Don’t even consider covering the
    bench with carpeting; you’ll lose so many parts in it that you could eventually shake it
    out and build a fusion reactor from what you find! Also, carpeting can build up static
    electric charges lethal to circuitry.
         Carpeting on the ground around the bench has its pros and cons. It’s easy to lose
    small parts in it, but it also helps prevent them from bouncing away into oblivion when
                         Chapter 2      Setting Up Shop: Tools of the Trade              7




                  Figure 2-1      The well-appointed workbench:
                  a little smaller than ideal, but it does the job


they fall. If you do choose to have a carpeted floor, pick a light color and as shallow and
tight a pile as possible. This is no place for a thick carpet with loose fibers.
     You’ll need modern, three-wire (grounded) electrical service at your bench. This
is critical for safety! A grounding adapter plugged into a 1920s two-wire outlet will
not do, even if you screw the adapter’s ground lug to the wall plate. Most of those
plates are not properly grounded, and a bad ground can get you killed in certain
circumstances.
     The current (amperage) requirement is not high for most service work. Your
scope and other instruments won’t eat a lot of power, and most benches can be run
quite safely using a single, modern 15-amp plug fanned out by a couple of hardware
store-variety power strips. Also, this arrangement has the advantage that all ground
points are at exactly the same voltage level, which helps prevent ground loops
(unwanted current between ground points). Again, be sure the strips are three-wire,
grounded types.
     Lighting is another very important factor that shouldn’t be ignored. While it
might seem obvious that the entire room should be brightly lit, that is not the most
productive approach, as it can actually make it harder to see small details that need
to be scrutinized and, therefore, brighter than their surroundings. Average lighting in
the room is adequate. What you need most is spot lighting, and the best solution is a
fluorescent light on a swing arm, as shown in Figure 2-1. If it has a magnifier, all the
better, but you’ll be wearing one anyway, so it’s not necessary.
     Forget about using an incandescent bulb; the heat it produces will cook your
hands, your face and the gadget you’re trying to fix. An inexpensive way to obtain the
8   How to Diagnose and Fix Everything Electronic

    necessary lighting is to get a swing-arm desk lamp and replace its incandescent bulb
    with an “eco bulb,” one of those now-ubiquitous spiral light bulb replacements. Be
    aware, though, that many eco bulbs have a rather yellowish tint and also put out a fair
    amount of ultraviolet light, so using one close to your eyes may not be comfortable.
    Plus, they operate at a high frequency and can emit significant short-range radio-
    frequency energy capable of interfering with some kinds of measurements or even the
    circuit under test. The old circular, bluish-white fluorescent lamp is still your best bet.


    Digital Multimeter
    A multimeter (pronounced “mull-TIH-mih-ter”) is a device that can test several
    electrical parameters. The most common and important quantities you’ll need to
    measure are voltage (volts), resistance (ohms) and current (amps or, more typically,
    milliamps, which are thousandths of an amp). The analog incarnation of this test
    device, recognizable by its big meter needle and multiple-stop selector knob, used to
    be called a VOM (volt-ohm-milliammeter). Now that the meters are digital, they’re
    usually called DMMs (Digital Multi Meters), but they do the same thing, except that
    the readout is numerical instead of something interpreted from the position of a
    meter needle.
         DMMs began as very expensive, high-end laboratory instruments, but they’re
    cheap now and pretty much all you can buy. The market positions have reversed, and
    VOMs have become the exotic technology, with a good one selling for considerably
    more than a digital. Hardware stores and RadioShack (a.k.a. TheShack) offer DMMs for
    around $20 to $50, and they’re on sale on occasion for as little as $5. Some, however,
    can still be in the range of $200 or more. The expensive ones may have the ability to
    test various other parameters like capacitance and inductance, but mostly what they
    offer are much higher precision and accuracy.
         Precision and accuracy are two different things. Precision is the fineness to which
    a measurement is specified, and accuracy is how truthful the measurement is. For
    instance, if I say, “It’s between 60 and 80 degrees outside,” and the actual temperature
    is 72 degrees, my statement is not very precise, but it’s quite accurate. If, however,
    I say, “It’s 78.69 degrees outside,” and it’s really 82 degrees, my statement is very
    precise but not at all accurate.
         So, for a DMM to specify that it measures voltages to three digits to the right of the
    decimal point, it has to have a basic accuracy of somewhere around a thousandth of
    a volt. Otherwise, those pretty digits won’t mean much! Who on Earth would build an
    instrument that displayed meaningless numbers?
         Makers of low-cost DMMs do it all the time. The digits make one manufacturer’s
    unit look more desirable than another’s, but the basic accuracy doesn’t support
    them. Does it matter? Not really, as long as you are aware of the limitations of the
    instrument’s basic accuracy, so you know what to ignore toward the right side of the
    display. In any event, all DMMs, even the cheapies, are both more precise and more
    accurate than any VOM ever was.
         Just how much precision do we need? For general service work, not a lot. When
    things break, they don’t do so in subtle ways. For example, if you’re checking the
                         Chapter 2      Setting Up Shop: Tools of the Trade               9

output of a 5-volt power supply, and your DMM reads 5.126 volts, that’s not cause for
concern. If it reads 3.5 volts or 7 volts, then perhaps you’ve found a problem! Bottom
line: you don’t need a $200 DMM. The $20 to $50 instruments will do fine.


Oscilloscope
Many hobbyists feel intimidated by the oscilloscope, but it is the best buddy any tech
can have. Repeat after me: “My scope is my friend.” Come on, say it like you mean it!
Once you get the hang of using one, you will love it, I assure you.
    The basic function of an oscilloscope is to generate a graph of voltage versus
time. As a spot sweeps from left to right across the screen at a constant rate, it also
moves up and down in relation to the incoming signal voltage, drawing a waveform, or
representation of the signal that shows you how the voltage is changing. The maximum
rate of change of the voltage amplifiers driving the vertical motion determines the
bandwidth, or how fast a signal the scope can display. Most scopes in our range of
interest have bandwidths of around 100 MHz (100 million cycles per second). They
also have two vertical input channels, meaning they can display two waveforms at the
same time. In Chapter 6, we’ll explore how to use a scope—it really isn’t hard—but first
you have to get one. There are several types.


Analog
This is the classic scope with a green CRT (cathode ray tube, a.k.a. picture tube). It
displays signals as they arrive and has no memory functions to store waveforms. It
doesn’t sample them, it doesn’t dice or slice them; it just shows them to you, plain and
simple. A classic analog scope is shown in Figure 2-2.
     Analog scopes have been available since around the 1940s, and they really got
good in the 1970s. Some are still being made today, though digital scopes have been
at the forefront of the marketplace for a decade or more. Newer is better, right? Not
always. The oscilloscope is a good example of an older analog technology being
superior in some ways to its replacement. For most general service work, an analog
scope is the simplest to use, and its display is the easiest to interpret. Further, it shows
details of the signal that digital scopes may miss.
     The lowest-end analog scopes have just one channel of input, and they lack
features like delayed sweep, a very handy function that lets you zoom in on any part
of a waveform you want and expand it for detailed viewing. Avoid them. There are
tons of great analog scopes with all the nice features on the used market at ridiculously
cheap prices, so there’s no need to skimp on the goodies. Make sure any analog scope
you buy has two channels (some have even more, but two are standard) and delayed
sweep. Look for two input connectors marked “A” and “B” or sometimes “Channel 1”
and “Channel 2,” indicating two vertical input channels. If you see a metal knob that
can be turned multiple times and slowly advances a number imprinted on it, or you
see “A and B” on the big knob marked “Sec/Div” or “Horiz Sweep,” or buttons marked
“B after A,” “B ends A,” or “B delayed,” then the unit has delayed sweep. Another tipoff
10   How to Diagnose and Fix Everything Electronic




                       Figure 2-2     Leader LBO-518 100-MHz analog
                       oscilloscope


     is a knob marked “Trace Sep.” If you’re still not sure, look up the model on the Internet,
     and you can probably find its specs or even download a free PDF of the manual.


     Digital
     Digital scopes are new enough that they all have delayed sweep and two channels,
     except for a few handheld models. Though some early examples used CRTs, modern
     digital scopes can be recognized by their shallow cases and LCD (liquid crystal
     display) screens (see Figure 2-3). With a digital instrument, you can grab a waveform
     and examine it in detail long after it has ceased. Thus, digital scopes are ideal for
     working on devices with fleeting signals you need to be able to snag that may zip by
     only once.
          Such is almost never the case in the kind of service work you’ll be doing. The
     vast majority of the time, you will be looking at repetitive signals that don’t have to be
     stored, and the limitations of a digital scope may get in your way.
          One significant limitation arises from the basic nature of digital sampling, or
     digitizing, in which a voltage is sampled, or measured, millions of times per second,
     and the measured value of each sample is then plotted as a point on the screen. Alas,
     real-life signals don’t freeze between samples, so digital scopes miss some signal
     details, which can result in a phenomenon called aliasing, in which a signal may be
     seriously misrepresented. (This is the same effect that causes wagon wheels in old
     Westerns to appear to rotate backward—the movie camera is missing some of the
                        Chapter 2      Setting Up Shop: Tools of the Trade           11




        Figure 2-3     Tektronix TDS-220 100 MHz digital oscilloscope




wheels’ motion between frames.) If the sampling rate is considerably faster than the
rate of change of the signal being sampled, aliasing won’t occur. The sampling rate goes
down, though, as you slow the sweep rate (speed of horizontal motion on the screen)
down to compress the graph and squeeze more of the signal on the screen. As a result,
when using a digital scope, you must always keep in mind that what you’re seeing
might be a lie, and you find yourself turning the sweep rate up and then back down,
looking for changes in the waveform suggestive of aliasing. It takes some experience
to be certain what’s on the screen is a true representation of the signal. Even so,
sometimes aliasing is unavoidable at lower sweep rates, limiting how much of the
signal you can view at once—a conundrum that never occurs with analog scopes.
    Another big limitation arises from the screen itself, and it also limits how much
you can see at one time. Unlike the continuously moving beam of the analog scope,
the digital scope’s display is made up of dots, so it has a fixed resolution, and nothing
can be shown between those dots. (That’s why the sampling rate goes down at lower
sweep rates; there’s no point in taking samples between dots, since there’s no place
to plot them anyway.) When examining complex waveforms like analog video signals,
the result is a blurry mess unless you turn the sweep rate way up and look at only a
small part of the signal. While an analog scope can show a useful, clear representation
of an entire field of video, a digital instrument simply can’t; all you see is an
unrecognizable blob.
    Probably the most profound difference between an analog and a digital scope is
that an analog instrument actually writes the screen at the sweep speed you select,
while a digital unit does not. A digital collects the data at that speed, but it updates
the screen much slower because LCDs don’t respond very fast. For many signals,
that’s fine, and it can even help you see some signal features that might be blurred by
repetitive overwrites on an analog screen.
    Sometimes, however, those overwrites are exactly what you want. When viewing
the radio-frequency waveforms coming from video and laser heads, for instance, you
need to evaluate the envelope, or overall shape of the waveform over many cycles,
12   How to Diagnose and Fix Everything Electronic

     not its individual waves. The overwriting and true-to-life writing speeds inherent in
     an analog scope make envelopes stand out clearly. Some envelopes can’t be viewed
     at all with a digital, because it misses too much between screen updates. You’ll see
     individual cycles of the waveform, but not their outer contour, unless you slow down
     the sweep rate so low that all you get is a featureless blur.
          On the plus side, digital scopes are naturals at measuring waveforms, not just
     displaying them. They can provide a numerical readout of peak voltage, frequency,
     time difference, phase angle, you name it. While all of those measurements can be
     done with an analog scope, the computer in that case is your brain; you have to do the
     math, based on what you’re seeing on the screen. With a digital scope, you position
     cursors on the displayed waveform and the scope does the work for you. Having quick
     and easy measurement of signal characteristics can greatly speed up troubleshooting.
          When choosing a digital scope, look for the sample rate and compare it to the
     vertical bandwidth. The sample rate should always be higher than the bandwidth
     so the scope can perform real-time sampling. The Tektronix TDS-220, for example
     (Figure 2-3), samples at 1 gigasample (billion samples) per second, with a bandwidth
     of 100 MHz. Thus, one cycle of the fastest waveform it can display will be broken into
     ten samples, which is pretty good. At a minimum, the sample rate should be four
     times the bandwidth.
          It is possible to sample repetitive waveforms at a rate slower than the bandwidth
     using a technique called equivalent-time sampling, in which each successive waveform
     is sampled at different points until the full representation is assembled. Equivalent-
     time sampling was developed when analog-to-digital converters were too pokey for
     real-time sampling of fast waveforms. It is an inferior technique, because developing
     an accurate representation requires the incoming signal to remain unchanged from
     cycle to cycle for as many cycles as it takes to assemble one. Plus, what you see is
     never a true picture of any one particular cycle. And, heck, it’s just plain slow. Avoid
     any scope depending on it to reach its bandwidth specs. Real-time sampling is the
     only way to fly.


     Analog with Cursor Measurement
     This is the best of both worlds: an analog scope capable of performing many of the
     measurements available in a digital instrument. This style of scope doesn’t digitize
     signals, thus avoiding all of the limitations associated with that process. Like a
     digital, though, it uses movable cursors to mark spots on the displayed waveform and
     calculate measurements. This is my favorite type of scope.


     Analog with Storage
     Before the advent of digital scopes, some analog units were made with special CRTs
     that could freeze the displayed waveform, enabling a crude form of signal storage.
     These scopes were expensive and always considered somewhat exotic. Digital storage
     has completely supplanted them.
                        Chapter 2      Setting Up Shop: Tools of the Trade            13

PC-Based
The PC-based scope uses your general-purpose computer as a display and control
system for a digitizing scope. It seems like a great idea, because you get a nice, big,
high-resolution screen, and you can use your computer’s keyboard and mouse to
control the features. Plus, PC scopes are cheaper, since you’re not paying for knobs
and an LCD. In practice, PC scopes are the worst option for service work. They’re
awkward to use and usually offer the lowest performance in terms of sampling rate.
I recommend you avoid them.


Buying an Oscilloscope
New scopes are fairly expensive. Expect to pay around $1000 for a 100-MHz instrument.
But why spend a lot when there are so many nice scopes on the used market for next to
nothing? It’s quite possible to get a good used scope for about $100.
     There are plenty of scope manufacturers, but the gold standard in the oscilloscope
world is Tektronix. Tek has dominated the scope market since the 1970s, and for good
reason. Many of its model 465 and 475 scopes from that era are still going strong,
more than 30 years later! If you find one of those models in good working order for
less than $100, it’s worthy of consideration. Much newer models are also available,
including the 2200 and 2400 series, and they’re pretty cheap too. Other good scopes
are made by Hitachi, Hewlett-Packard, B&K Precision and Leader. Fluke, famous for
its DMMs, makes a series of handheld digital scopes, too, as does Tek. They’re a tad
pricey, though, and a bit harder to use, thanks to their extensive use of menus, since
they have little room for knobs.
     Where do you find a used oscilloscope? Good old eBay is loaded with them, and
they show up now and then on craigslist.org. Try going to your area’s hamfest, a
periodic swap meet put on by ham radio operators and electronics aficionados, and
you’ll see plenty of scopes. Just be sure you can check that the instrument works
properly before you plunk down your cash. Read Chapter 6 first so you’ll know how to
test the scope. Look for a nice, sharp trace and no lines burned into the display tube, if
the scope uses one.
     Along with the scope, you’ll need a pair of probes. Scope probes are more than
just pieces of wire; they have voltage dividers in them and are specialized devices
designed to permit accurate signal measurement. Most divide the incoming voltage
by 10 (you’ll see why later) and are called 10X probes. Some have switches to remove
the division, and are known as switchable 10X/1X probes (see Figure 2-4). Like scopes
themselves, probes are rated by bandwidth, and the high-end ones can cost a lot.
Luckily, 100-MHz probes can be found on eBay brand new for around $15 each. New
probes will include slip-on covers with handy hooks on the ends. If you buy used
probes, try to get the hooks too.
14   How to Diagnose and Fix Everything Electronic




                      Figure 2-4     Oscilloscope probe, 10X/1X
                      switchable




     Soldering Iron
     Soldering irons come in various shapes and sizes, and you’ll probably wind up with
     more than one. The smallest, with heating elements in the 15-watt range, are great
     for getting into very tight spots and working on tiny surface-mount parts, at least on
     boards assembled with low-temperature solder. Those irons don’t generate enough
     heat to solder a power transistor, though. The largest irons, usually pistol-shaped
     guns with elements of 100 watts or more, put out lots of heat and have sizable tips to
     transfer it to the part you’re soldering. Those big guns can be real life-savers, but you
     sure don’t want to try soldering minuscule parts with them. Even if you could fit the
     tip where you needed it, the excessive heat would destroy the part and probably the
     circuit board as well.
          The best choice for general soldering work on printed circuit boards is an iron
     with a medium-sized tip and a heating element in the range of 40 to 70 watts. Melting
     leaded solder requires a tip temperature of about 375–400 °F. The newer, lead-free
     variety needs a much hotter tip, in the area of 675–700 °F. Some inexpensive irons
     in the 20-watt range are about the same size, but steer clear of those. Supplying
     inadequate heat can cause lots of harm; you may easily pull up copper traces and
     severely damage the printed circuit board if things aren’t hot enough, especially when
     removing components. Plus, not using enough heat can result in “cold” solder joints
     that don’t transfer electrical energy properly, causing your repair to fail.
          Many inexpensive irons of medium size plug directly into the wall. This is not
     the best way to go, as it may expose the circuitry you’re soldering to small leakage
                         Chapter 2      Setting Up Shop: Tools of the Trade            15

currents from the AC line, and having the cord go off to a power strip can be awkward
as you move the iron around. Finally, should you accidentally lay the iron on the cord
and melt through the insulation, you’ll cause a short directly across the AC line, which
is likely to be spectacular and unpleasant, and possibly dangerous. Don’t laugh, it
happens!
     A far better solution is an iron that plugs into a base unit with a step-down
transformer. This kind of setup runs the heating element at low voltage and isolates
the tip from the AC line. The base unit gives you a nice stand to hold the iron and
a sponge for wiping the tip, too. Some bases have variable heat controls, and some
even have digital temperature readouts. Before the age of lead-free solder, I never
found such things to be useful, because the heat pretty much always needed to be
turned all the way up on smaller irons anyway. These days, a variable-heat soldering
station that can hit the temperatures required for lead-free soldering is well worthy of
consideration.
     Numerous companies make soldering irons, but two make the nicest, most durable
irons, the ones found in service shops: Weller and Ungar. These irons can cost from
$50 to more than $100, but they are worth every penny and will last for many years.
Your soldering iron is usually the first thing you turn on and the last you turn off, so it
will run for thousands of hours and needs to be well-made. Don’t be tempted by those
$20 base-unit irons flooding the hobbyist market. They just don’t hold up, and you’ll be
needing a new one before you know it.
     As with scopes, good used irons often show up at great prices at hamfests.
Wherever you get your iron, plan on buying a spare tip or two. Tips wear out and
become pitted and tarnished to the point that they no longer transfer heat well, so
they must be replaced every few years. The heating elements can go bad too, but it’s
rare; I’ve seen them last for decades on the good irons.
     The big guns are cheap, typically under $20, so buy one. There will be situations
in which you will be very glad you did.


Plastic-Melting Iron
Sooner or later, you’ll want to melt some plastic to repair a crack or a broken post. It’s
unhealthy to breathe in molten plastic fumes, but we all melt the stuff now and then,
being as careful as we can with ventilation. If you’re going to melt plastic, don’t do
it with the same iron you use for soldering! The plastic will contaminate and pit the
tip, making it very hard to coat it with solder, or tin it, for subsequent soldering work.
Instead, pick up a cheap iron in the 20- to 30-watt range and dedicate it for plastic use.
For this one, you don’t need a base unit or any other fancy accoutrements. You should
be able to get a basic iron and a stand to keep it from burning its surroundings for
around $10.


Solder
Traditional solder is an alloy of tin and lead with a rosin core that facilitates the
molecular bond required for a proper solder joint. In the past, the alloy was 60 percent
16   How to Diagnose and Fix Everything Electronic

     tin, 40 percent lead. More recently, it has shifted to 65 percent tin, 35 percent lead.
     This newer type of solder is better suited to the lower temperatures associated with
     tiny surface-mount parts, and it’s getting hard to find the old 60/40 stuff anymore.
     The old proportions were better for the higher-heat environment of power transistors
     and voltage regulators. If you can find some 60/40, it’s worth getting. If not, you can
     live with the newer variety. Lead is a toxic metal, and lead-free solder has become
     available and is widely used in the manufacture of new electronics, to comply with
     the legal requirements of some countries and states. The European “Restriction of
     Hazardous Substances” or RoHS standard is gradually being adopted around the world.
     All products displaying the RoHS mark are made with lead-free solder. You can buy
     the stuff for your repair work, but I recommend against doing so, because it’s hard
     to make good joints with it. It doesn’t flow well, and cold joints often result. Plus, the
     higher heat required to melt it invites damage to the components you’re installing.
           Lead vaporizes at a much higher temperature than that used for soldering. The
     smoke coming off solder is from the rosin and does not contain lead you could inhale.
     Handling solder, however, does rub some lead onto your hands. So never snack or
     touch food while soldering, and always wash your hands thoroughly after your repair
     session ends.
           There is a variety of solder, found in hardware stores and intended for plumbing
     applications, with an acid core instead of rosin. Never use acid-core solder for electronics
     work! The acid will corrode and destroy your device. By the same token, the rosin flux
     paste used with acid-core solder is not needed for normal electronics solder, because
     rosin is already in its core.
           Solder comes in various diameters. A good choice for normal work is around 0.03
     inches. Very small-diameter solder, in the 0.01-inch range, can be useful now and then
     when working with tiny parts, but not often. For most jobs, it’s so undersized that you
     have to feed it into the work very fast to get enough on the joint, making it impractical
     to use. My own roll of the skinny stuff has been sitting there for a decade, and most of
     it is still on the roll.
           Solder is like ketchup: you’ll use a lot of it. Buy a 1-pound roll, because it’s a much
     better bargain per foot than those little pocket packs of a few ounces. A pound of
     solder should last you a good few years.


     Desoldering Tools
     Removing solder to test or replace parts is as vital to repair as is soldering new ones
     to the board. Desoldering ranges from easy to tricky, and it’s a prime opportunity for
     doing damage to components and the copper traces to which they’re attached. Fancy
     desoldering stations with vacuum pumps can cost considerably more than even top-
     end soldering irons. For most service work, though, you don’t need anything exotic.
     There are some low-cost desoldering options that usually do the trick.

     Solder Wick
     One of the best desoldering tools is desoldering braid, commonly called solder wick.
     It’s made of very fine copper wire strands woven into a flat braid. Usually, it is coated
                        Chapter 2      Setting Up Shop: Tools of the Trade           17

with rosin to help solder flow into it. (You may run across some cheap wick with no
rosin. Don’t buy it; it doesn’t work.) Wick can be purchased in short lengths on small
spools from RadioShack and various mail-order companies. Electronics supply houses
offer it in much longer lengths on bigger spools. As with solder, the bigger spools are
the far better deal. Always be sure to keep some wick around; it’s some of the most
useful stuff in your workshop.

Bulbs
Another approach to solder removal is to suck it up with a rubber solder bulb, or
solder sucker. Bulbs come in two forms: stand-alone and integrated with a soldering
iron (see Figure 2-5). Both have their uses, but with the integrated type, you’re limited
by the heating power of the built-in iron, which is usually not especially strong. Stand-
alone bulbs are cheap, so get one even if you also get an integrated type.

Spring-Loaded Solder Suckers
One of the handiest solder removal tools, the spring-loaded solder sucker is another
inexpensive option. These bad boys have a fast, almost violent action, and are a bit
harder to control than bulbs. They suck up a lot of solder in one motion, though.
Get one.

Vacuum Pump Desoldering Irons
These are like integrated bulb desoldering units, except that the suction is provided by
a vacuum pump instead of a bulb. The pros use these, and they’re fast and powerful,
but they’re expensive. If you can snag a good used one at a hamfest, go for it. Just be
sure the heating and vacuum systems work properly. The vacuum portions are prone
to problems and worn parts, because molten solder flows through them.




                 Figure 2-5     Bulb-type desoldering iron
18   How to Diagnose and Fix Everything Electronic

     Chip Quik
     This is a special low-temperature solder alloy and flux kit used for desoldering
     surface-mount parts. When melted into the existing solder, the alloy keeps it molten
     at low temperatures, allowing you to get lots of pins hot enough simultaneously to
     remove even high-density chips with dozens of leads.


     Hand Tools
     The range of available hand tools seems practically infinite. Most likely, you’ll build
     up a significant collection of them as the years go by. My own assortment fills several
     drawers. While nobody needs six pairs of needlenose pliers, there is a core set of tools
     necessary for disassembling and reassembling the items to be repaired.

     Screwdrivers
     Today’s gadgetry uses a wide range of types and sizes of screws. Some of the screws
     are incredibly tiny. A set of jeweler’s screwdrivers is a necessity. Both Phillips and
     flatblade screws are used, though Phillips types dominate. More and more, hex and
     Torx heads are showing up too (see Figure 2-6). The latter shapes started out as a
     way to prevent consumers from opening their gadgets, but the drivers have gradually
     become available, defeating that objective. In response, newer types have come along.
     One of the most recent is the Trigram, which looks like a center point with three lines
     radiating out toward the perimeter of the screw head. In time, those drivers will be
     easier to find as well.
          Get a good selection of small drivers in all these form factors. At the very least,
     get Phillips, flat blade and Torx screwdrivers. Pick up a few medium-sized Phillips
     and flathead drivers too. You’ll use the smaller ones much more often than the larger
     ones, but it pays to have as many sizes as you can find. Really big ones are rarely
     needed, though.




                          Figure 2-6    Hex and Torx driver tips
                        Chapter 2       Setting Up Shop: Tools of the Trade           19

Cutters
Diagonal cutters, or dikes, are used to clip the excess lead lengths from newly installed
components. Most techs also use them to strip insulation off wire. Again, smaller
beats bigger. Get a couple of pairs of these things, because they tend to get bad nicks
in their cutting edges and gradually become useless. A pair of dikes shouldn’t cost
more than about $7. Oh, and be sure the handles are insulated. They usually are.

Needlenose Pliers
A pair of needlenose pliers is essential for grabbing things, reaching into cramped
spots, and holding parts steady while you solder them. A length of 2 or 3 inches from
the fulcrum to the tips is about right. Any shorter and they may not reach where
you need them. Any longer and they’ll probably be a bit too flexible, reducing their
usefulness when twisting is required. Unlike cutters, needlenose pliers rarely wear
out or need replacement. Still, get two pairs so you can hold one in each hand and use
them at the same time. You’ll need to do that now and then.

Hemostats
They’re not just for surgeons anymore! Hemostats are much like needlenose pliers,
except that they lock, providing a firm grip without your having to keep squeezing the
handle. Some have corrugated gripping ends, while others are smooth. Get one pair of
each style. These things are indispensable for pulling a component lead from a board
while heating the solder joint on the other side. They’re great for installing new parts,
too. You can find hemostats at most electronics and medical supply houses.


Magnifier
With the size of today’s electronics, human eyes have hit their resolution limit for
comfortable close-up work. It’s essential that you have some magnification. Even if
your spot lamp has a magnifying lens, you’ll still need a head-worn magnifier, because
the lamp will get in your way when it’s placed between your face and a small gadget.
Glass is better than plastic, which gets scratched and can even melt when situated
very near a soldering iron. Be sure the magnifier you choose can be flipped up out of
the way, because sometimes you need to step back a bit from the work and take in
a longer view. If you wear glasses, you may need to get a magnifier with adjustable
focus to keep it compatible with your eyewear.


Clip Leads
Frequently, testing involves making temporary connections. For that, nothing beats a
batch of clip leads, which are wires about a foot long with alligator clips at both ends.
Get at least ten, making sure that the clips are small and have rubber insulating covers.
20   How to Diagnose and Fix Everything Electronic

     You can buy the clips and make them yourself, but assembled ones are readily available
     and inexpensive. Just be aware that the premade ones are usually not soldered; the
     wires are merely crimped to the clips. After some use, they break inside the insulation,
     which leads to some head scratching when a connection doesn’t produce the expected
     results. You can never completely trust the integrity of a clip lead. The quick test is to
     pull the lead taut while holding on to the clips. If broken, one end will fall apart, after
     which you can solder it on. In time, you’ll wind up soldering all of them.


     Swabs
     Cotton swabs are very useful in the shop. Get the kind with paper sticks, not plastic
     ones. The paper type can be bent into shapes that will let you poke them into odd
     corners. If you can find them, also get some chamois swabs. Unlike the cotton type,
     chamois swabs don’t leave little fibers behind. For some uses, especially cleaning
     video heads on VCRs and camcorders, the fibers can be problematic, and it’s even
     possible to break a video head if the fibers get snagged on it while you clean.


     Contact Cleaner Spray
     There are many brands of spray, each claiming superiority, but they all do pretty
     much the same thing: remove oxidation and dirt from electrical contacts. One of
     the more popular brands is DeoxIT. RadioShack’s spray is called TV-Tuner/Control
     Cleaner & Lubricant. Get a can or two. It’s handy stuff and you’ll be using it,
     especially if you work on older gear.


     Alcohol
     Alcohol can be very useful in cleaning tape paths and heads. Use isopropyl alcohol,
     and look for the highest percentage you can find. The 70-percent solution sold in
     drugstores is 30 percent water, which is bad for electronics. Some stores sell 91-percent,
     which is much better, and I’ve run across 99-percent on occasion. Don’t use ethyl or
     any other type of alcohol. Be aware that all alcohols can damage some types of plastic
     rather badly. When working with alcohol, keep it away from plastic casings, LCD
     screens and control panels.


     Naphtha
     Sold in little yellow bottles as “cigarette lighter fuel” at grocery stores, and in bigger
     containers as “VM&P Naphtha” at hardware stores, naphtha is an amazing solvent
     that will effortlessly remove grime, sticker adhesive, solder rosin, tobacco tar and
     other general filth from just about any surface. I’ve never seen it harm plastic,
     either, not even LCD screens. It’s used by dripping a very small amount on a tissue,
     paper napkin or swab, and then gently rubbing the surface to be cleaned. Naphtha is
     seriously flammable, so never use it on anything to which power is applied, or near
                         Chapter 2      Setting Up Shop: Tools of the Trade             21

flame or a hot soldering iron. It’s best to use gloves, too, to keep it off your skin. Make
sure of proper ventilation, because it evaporates quickly and should not be inhaled.
Even if you buy a big can of it, also buy one of the little yellow bottles so you can
squirt out tiny doses as needed. It takes very little naphtha to do the job, and you can
always refill the bottle from the big can outside your house later on. Keep naphtha
containers tightly closed so evaporated fumes won’t build up and become hazardous
in biological or fire hazard terms.


Heatsink Grease
This silicone-based grease is used between transistors, voltage regulators and other
heat-producing parts and their metal heatsinks. It fills in the tiny gaps between
imperfect surfaces, helping transfer heat from the part to the heatsink. Even when
mica insulators are used, heatsink grease is still required for most parts. (The
exception is an installation using a special rubber heat transfer gasket; most of those
do not require grease.) When you replace a heatsinked part, you’ll need the grease;
omitting it will result in an overheated component that will quickly fail. A small tube
of heatsink grease lasts a very long time, as only a thin film is required, and too much
grease can actually reduce heat transfer.
    Silicone grease is inappropriate for use with microprocessors and graphics chips.
These hot-running parts require special silver-bearing grease, which you can find at
computer stores and mail-order suppliers.


Heat-Shrink Tubing
This stuff looks like ordinary plastic tubing, but it has a wonderful trick up its synthetic
rubber sleeve: it shrinks in diameter when you heat it up, forming itself around joints
and damaged insulation spots in wires. It’s much more permanent than electrical
tape, which tends to get gooey and let go after awhile. Get some lengths of heat-shrink
tubing in various small diameters.


Electrical Tape
Despite its impermanence, electrical tape still has uses in situations where tubing
won’t fit or can’t be slipped over what needs to be insulated. Plus, for extra insulative
peace of mind, you can wrap a connection in tape and then put tubing over it.


Small Cups
If you eat yogurt or pudding, start saving the little plastic cups. However you obtain
them, those cups are incredibly useful for temporary storage of screws and other
small parts as you disassemble machines. Make sure the cups fit into each other.
Most will.
22   How to Diagnose and Fix Everything Electronic

     Internet Access
     Not that long ago, a significant stack of reference books was required for looking up
     transistor types, cross-referencing replacement components and finding disassembly
     hints and diagrams for various products. Now we can do all that and more on the
     Internet! Some sites charge for schematics, but you can find some free ones. Even if
     you have to pay a little bit for the diagram, it may be well worth it. There’s plenty of
     free info out there about how to take apart certain products without breaking them,
     too. Having a computer nearby with Net access is really handy.



Nice-to-Haves
     Here are some items that can help you get repair work done more easily. You can live
     without ’em, but you might want to add some to your arsenal as time goes on.


     Digital Camera
     How’s your memory? If it’s imperfect, like most of ours, a digital camera can save
     your rear end when you look up from the bench and realize you’ve removed 35 screws
     from four layers of a laptop, and you’re not sure where they all go. And what’s that
     funny-looking piece over there? The one you took off three days ago, just before you
     answered the phone and took the dog for his emergency walk? Take pictures as you
     disassemble your devices. Use the macro lens as necessary to get clear close-up shots;
     a blurry photo of a circuit board does you little good. Make sure you can see which
     plug went in which connector, and what the shield looked like before you removed
     it. Experiment a bit with the flash, too, and the angles required to get decent shots
     without too much glare.


     Power Supply
     Unless it has its own AC power supply, your repair item runs either on batteries or
     from an AC adapter. AC adapters themselves fail often enough that you can’t assume
     the adapter isn’t the problem. So, running the device under test from a variable
     power supply can really help. The most important issue when choosing a supply is
     how much current it can provide. While a little pocket radio might eat around 50 ma
     (milliamps), a power amplifier or radio transmitter may require hundreds of times as
     much current. For most work, if you have 5 amps available, you’re covered.
          There are some fancy laboratory-grade power supplies with digital metering,
     ultra-precise regulation, and price tags to match. You don’t need one. Any decent,
     hobby-grade supply will do.
          Many of the items that demand high current are for use in the automotive
     environment, so a 12-volt (more typically 13.8 volts, the actual voltage of a car with
     its engine running) supply with 10 amps or more is great to have as well. That one
     doesn’t need to be variable, since all auto gear runs on the same voltage.
                         Chapter 2       Setting Up Shop: Tools of the Trade             23

Transistor Tester
Although it’s possible to test many characteristics of transistors with a DMM, some
failure modes, like excessive reverse leakage (when a small current can flow backward
through a defective transistor’s junctions), aren’t easy to find that way. Dedicated
dynamic transistor testers use the transistor under test as part of an oscillator, measuring
how the part behaves with real signals applied to it. Under those conditions, you can
measure the approximate gain, high-frequency cutoff point and leakage. Many testers
can check various transistor types, including MOSFETs (metal-oxide-semiconductor
field-effect transistors), junction FETs (field-effect transistors) and standard bipolars.
     Basic transistor testers of this type are inexpensive and a great addition to your
bench setup. They’re fairly easy to make, too, and you can find diagrams in hobbyist
magazines.


Capacitance Meter
Capacitors, especially the electrolytic type used in power supplies as filtering elements
to smooth the output power, are some of the most trouble-prone components of all. In
addition to suffering complete failures like opens and shorts, capacitors can gradually
lose their ability to store energy. Worse, their internal equivalent series resistance (ESR)
can rise, in which case the capacitor will measure just peachy keen on a meter, but
will act like it has a big resistor between it and the circuit when it’s in use. Excessive
capacitor ESR is one of the most common causes of oddball circuit behaviors.
     Some DMMs have built-in capacitance measurement, but ESR meters are still on
the expensive side. You can get by without one, though; in Chapter 11 we’re going to
explore how to evaluate capacitors in operating circuits, using our good friend the
oscilloscope.


Signal Generator
More useful for servicing analog equipment than digital, a signal generator lets you
inject a test signal into a device’s signal-processing stages to see whether doing so
causes the expected effect. With today’s digital devices, it’s not something you’ll
use very often, but it has some application in simulating the clock oscillators that
drive digital circuitry. Many products, like MP3 players, have both analog and digital
sections, and a signal generator can come in handy with those if the audio circuitry
malfunctions.
     The generators are called function generators when they have the ability to create
different kinds of waveforms, such as sine, triangle and square waves. While sine
wave–only generators are usually segregated by frequency band, either audio or
radio, function generators may have a wide range encompassing both, though they
don’t offer high-frequency ranges anywhere near those of radio-only generators.
Many function generators operate from a couple of Hz (hertz, or cycles per second)
to around 2 MHz (megahertz, or millions of cycles per second), while radio-frequency
(RF) generators may reach hundreds of megahertz.
24   How to Diagnose and Fix Everything Electronic

     Frequency Counter
     A frequency counter does just that: count the frequency of a signal. It does so by
     opening a gate for a precise period of time and counting how many cycles of the
     signal get through before the gate closes again. A counter is most useful when the
     frequency of a circuit’s oscillator needs to be adjusted accurately. This is rarely the
     case with digital devices like cameras and computers, but it can be critical when
     calibrating the master oscillators that control the tuning of radio receivers and
     transmitters.
          If you’re considering getting a counter, look more at its low-frequency capabilities
     than at the high end, unless you plan to work on UHF or microwave systems. Many of
     the inexpensive counters that can hit 1 GHz (gigahertz, or billions of cycles per second)
     are optimized for radio work and have gate times too short to count audio frequencies
     accurately. (The slower the signal frequency to be counted, the longer the gate has
     to stay open to let enough cycles through for a proper count.) Oh, and counters are
     another product category, like DMMs, that may display lots of meaningless digits.
     Especially with the cheapies, there can be a long string of numbers to be taken with
     a significant grain of sodium chloride.
          Frequency counters are capable of counting regular, continuous signals only;
     they’re useless with complex, changing ones. To extract frequency information from
     those, you need…yup, a scope. See, I told you that darned scope was your friend!
          Speaking of scopes, you’ll need an extra scope probe for your counter unless you
     want to share one with the scope. Get one with a 10X/1X switch, as that is especially
     handy for counter use.


     Analog Meter
     The moving meter needle of an old-fashioned analog VOM offers some info to the
     trained eye that a modern DMM can’t. Slowly fluctuating voltages, which you might
     encounter with, perhaps, a bad voltage regulator or a circuit pulling too much current,
     are even easier to see with a VOM than with an oscilloscope. Little voltage dips or
     spikes cause a characteristic bounce of the needle that’s very informative, too. You
     can even get a rough estimation of an electrolytic capacitor’s condition with an analog
     ohmmeter by watching the needle quickly rise and then slowly drop. With a DMM,
     such changes are just rapidly flashing numbers impossible to interpret.
           A special type of VOM is known as a VTVM, for vacuum tube volt meter. A VTVM
     works like a VOM, but it contains an amplifier, making it considerably more sensitive
     to small signals and much less likely to steal meaningful amounts of current from
     the circuit under test, or load it down. VTVMs go back a long way, from before there
     were transistors, and early ones really did use vacuum tubes. Later models substituted
     the tubes with a very sensitive type of transistor called a field effect transistor (FET).
     Those were known as FET-VOMs, but most people continue to call any amplified
     analog meter a VTVM, whether it has a tube or not. If you can find a FET-VOM, you
     might want to snap it up, because they’re getting rare. True VTVMs are very, very old,
     and replacements for those small tubes are hard to find, but some working ones are
     still out there.
                        Chapter 2      Setting Up Shop: Tools of the Trade           25

     Decent VOMs, FET-VOMs and VTVMs occasionally turn up at hamfests. As long
as it works, an old VOM is as good as a new one; there’s not much in it to go wrong.
If the meter needle moves without getting stuck and the selector switch works, you
should be good to go. True VTVMs may run on AC power or on batteries, but the
battery-operated units require cells nobody makes anymore, so avoid those meters.
VOMs and FET-VOMs use batteries that may sit in them for years, so check inside the
battery compartment to make sure an old cell hasn’t leaked and corroded the contacts.
Amplified meters (VTVMs and FET-VOMs) have a lot more in them than do VOMs,
too, so it’s best to test their functions before buying.
     Many companies have made VOMs, but the best oldies were made by Simpson,
which also made the best VTVMs. Some of those ancient Simpsons still go for real
money, and they’re worth it. You pretty much have to shoot them when you don’t
want them anymore. Triplett was another company that made great meters.


Isolation Transformer
It used to be that most AC-powered gear had a linear power supply, which shifted the
incoming voltage down to a lower one through a transformer operating at the 60-Hz
line frequency. The transformer, an assembly of two or more coils of wire on an
iron core, had no electrical connection between its input and output; the energy was
transferred magnetically. This arrangement helped with safety, because it meant that
the circuitry you might touch was not directly connected to the house wiring, and
thus couldn’t find a path to ground through that most delicate of all resistors, you.
Unfortunately, to move a lot of power required a big, heavy transformer.
     Today’s switching power supplies, or switchers (see Chapter 14), chop the incoming
power into fast bursts to push lots of energy through a small, light transformer. They’re
much more dangerous to work on, because some of their circuitry is connected directly
to the AC line, and it may have several hundred volts on it—and often it’s the section
that needs repair.
     An isolation transformer is just a big, old-style AC line transformer into which you
can plug your device. The transformer has a 1:1 voltage ratio, so it doesn’t change the
power in any way, but it isolates it from the AC line, making service of switchers a
lot safer. If you’re going to work on switching power supplies while they’re connected
to the AC line, you must have an isolation transformer. Many times, you can fix
switchers while they’re unpowered, so having an iso transformer is optional. Just
don’t ever consider working on a live switcher without one. Seriously! You don’t want
the power supply to wind up being the only thing in the room that’s live, if ya know
what I’m saying.


Stereo Microscope
With electronics getting smaller and smaller, even a head-worn magnifier may not
be enough for a comfortable view. More and more, techs are using stereo (two-eye)
microscopes to get a good, close look at solder pads on grain-of-salt–sized components.
When you choose a microscope, get one with low magnification power. You’re not
26   How to Diagnose and Fix Everything Electronic

     trying to see bacteria on the parts! 10X to 20X should be more than enough. Anything
     higher and you probably won’t even be able to recognize the component. A mono
     (one-eye) microscope can be used, but having the depth perception that comes with
     stereo vision can really help, especially if you’re trying to solder or desolder under
     the microscope. To find a microscope, check eBay. Sometimes they go for surprisingly
     affordable prices.
         Video-camera microscopes using a computer for display are becoming available,
     often for less than traditional optical microscopes. If you have room for a laptop on
     the bench, they’re worth considering.


     Bench Vise
     Wouldn’t it be wonderful to be gifted with three hands? If you have only the standard
     two, you may find it difficult to hold a circuit board while pulling a component lead
     from one side and heating the solder pad on the other. Check out the PanaVise and
     similar small vises designed for electronics work. Some offer attachable arms perfect
     for gripping the edges of circuit boards, and they let you swivel the board to whatever
     angle you need.


     Hot-Melt Glue Gun
     A small glue gun can help you repair broken cabinet parts, and a dab of hot-melt glue
     is also great for holding wires down. Manufacturers sometimes use it for that, and you
     may have to remove the glue globs to do your work. Afterward, you’ll want to replace
     the missing globs.


     Magnet on a Stick
     The first time you drop a tiny screw deep into a repair project and it won’t shake
     out, you’ll be glad you bought this tool. It’s useful for pulling loosened screws out of
     recessed holes, too. Get one that telescopes open like a rod antenna. Just keep the
     darned tip away from hard drives, tape heads, video head drums and anything else
     that could be affected by a strong magnetic field. Also, keep in mind that the metal
     rod could contact voltage, so never use the tool with power applied, not even if the
     device is turned off. Charged electrolytic capacitors can impart a jolt after power is
     disconnected, so keep away from their terminals too. See Chapters 3 and 7 for more
     about capacitors.


     Cyanoacrylate Glue
     Also known by the trademarks Super Glue and Krazy Glue, instant adhesive can
     be useful on some plastic parts. It’s strong, but it has poor shear strength and is not
     terribly permanent, so it shouldn’t be used for repair of mechanical parts that bear
                         Chapter 2      Setting Up Shop: Tools of the Trade               27

stress, as the repair will not last long. This type of glue is handy for holding things
together while you fasten them with other, more permanent means. Just be aware
that it outgases a white film as it hardens that is tough to remove, so keep it away
from lenses, display screens and other surfaces where that might be a problem.


Component Cooler Spray
This stuff is colder than a witch’s, um, iced tea, and it’s used to put the deep freeze
on suspected intermittent components, especially semiconductors (diodes, transistors
and IC chips). While it might seem primitive to blast parts instead of scoping their
signals, doing so can save you hours of fruitless poking around when circuits wig out
only after they warm up. One good spritz will drop the component’s temperature by
50 degrees or more and can reveal a thermal intermittent instantly, returning the
circuit to proper operation for a few moments until it heats up again.


Data Books
Although the Internet offers lots of great service-related information, some important
tidbits are still more easily found in a good old data book. Transistor cross-reference
data, which you use when you need to substitute one transistor type for another
because you can’t get the original type, is most easily looked up in a book. So is pinout
data for various ICs, voltage regulators and varieties of transistors.
    Motorola, National Semiconductor and ECG used to give away reference books,
but these days you’ll probably have to buy them from electronics supply houses.
At the least, consider getting a transistor substitution book. Of course, if it’s offered
as a CD-ROM or a downloadable PDF file, that’s even better, assuming you have a
computer near your bench.


Parts Assortment
Having a supply of commonly used components is quite handy. You can strip old
boards for parts, but it’s time-consuming, and you wind up with very short leads
that may be difficult to solder to another board. Plus, your stash will be hit-or-
miss, with big gaps in parts values. Consider buying prepackaged assortments of
small resistors and capacitors or going to a hamfest and stocking up for much less
money. There, you’re likely to find big bags of caps, transistors, chips, resistors, and
so on, for pennies on the dollar. Avoid buying transistors and chips with oddball
part numbers you don’t recognize, because they may be house numbers, which are
internal numbering schemes used by equipment manufacturers. Those numbers are
proprietary, and there’s no way to determine what the original type number was.
Thus, you can’t look up the parts’ characteristics, making them useless for repair
work. Resistors and capacitors, luckily, almost always have standardized markings,
and you can easily measure them if you have the appropriate meters.
28   How to Diagnose and Fix Everything Electronic

           Don’t bother buying ceramic disc capacitors, because they almost never go bad,
     so you aren’t likely to need any. If you do run into a suspicious one, you can pull its
     replacement off a scrap circuit board. Instead, focus on resistors, transistors, voltage
     regulators, fuses and, especially, electrolytic capacitors.
           Electrolytic capacitors are the cylindrical ones with plastic sleeves around them
     and markings like “10µf 25VDC.” There are many varieties of ’lytics, but some are
     pretty common, and substitution of similar but not identical parts is feasible in many
     instances. (We’ll explore how to do it in Chapter 12.) Get an assortment of caps in
     the range of 1 to 1000 µf (microfarads), with voltage ratings of 35 volts or more. The
     higher the ratings, both capacitive and voltage, the larger the cap. While it’s fine to
     replace a cap with one of a higher voltage rating, get some rated at lower voltages too,
     in case there isn’t room on the board for the bigger part.
           Stocking up on transistors is tricky because there are thousands of types. Small-
     signal transistors, which don’t handle a lot of current, are not hard to substitute, but
     power transistors, used in output stages of audio amplifiers and other high-current
     circuits, present many challenges, and they’re usually the ones that need replacement.
     Still, small transistors are very cheap—in the range of 5 to 25 cents each—and it’s worth
     having some around. Get some 2N2222A or equivalent, along with some 2N3906. You
     may find hamfest bags of parts with similar numbers that start with MPS or other
     headers. If the number portion is 3906 or 2222, it’s pretty much the same part and will
     do fine.
           Diodes and rectifiers are frequent repair issues, so it pays to have some on hand.
     The only difference between a diode and a rectifier is how much power it handles.
     Small-signal parts are called diodes, and larger ones made for use in power supply
     applications are dubbed rectifiers. Look for 1N4148 and 1N914 for the small fry, and
     1N4001 through 1N4004 for the big guns.
           The bridge rectifier, which integrates four rectifiers connected in a diamond
     configuration into one plastic block with four leads, is commonly used in power
     supplies. It’s a power-handling part that fails fairly often. Get a few with current
     ratings in the 1- to 5-amp range and voltage ratings of 150 to 400 volts.
           To house components, most of us use those metal cabinets with the little plastic
     drawers sold at hardware stores. Sort resistors and capacitors by value. If you have too
     many values for the number of compartments in the drawers, arrange the parts into
     ranges. For example, one compartment can hold resistors from 0 to 1 KΩ (kilohm, or
     thousand ohms), while the next might contain those from just above 1 KΩ to 10 KΩ.
     Once you learn to read the color code (see Chapter 7), plucking the part you need
     from its drawermates is easy.


     Scrap Boards for Parts
     Despite what I said about stripping old boards, you do want to collect carcasses for
     parts. No matter how large your components inventory is, the one you need is always
     the one you ain’t got! An old VCR or radio can provide a wealth of goodies, some of
     which are not easily obtained at parts houses, especially at 11:30 p.m. on a Sunday,
     when your hours of devoted sleuthing have finally unearthed the problem—at least
                            Chapter 2       Setting Up Shop: Tools of the Trade             29

   you think so—and you would sell parts of your anatomy for that one darned transistor,
   just to see if it really brings your patient back to life.
        If you have room, it’s easy to pile up dead gadgetry until your spouse, conscience
   or neighbors intervene. You’re highly unlikely ever to need cabinet parts, because
   they won’t fit anything beyond the models for which they were made, so saving entire
   machines is somewhat pointless and inefficient. The better approach is to remove
   circuit boards, knobs and anything else that looks useful, and scrap the rest. Don’t
   bother stripping the boards; just desolder and pull off a part when you need it. If the
   leads are too short, solder on longer ones.



Wish List
   For most service work, you can easily live without the following items, but they make
   for good drooling. Some advanced servicing requires them, but not often.


   Inductance Meter
   This meter reads the inductance value of coils, which seems like something quite
   useful, right? It’s really rare, though, for a coil to change its inductance without failing
   altogether. Usually, the coil will open (cease being connected from end to end) from
   a melted spot in the wire, as a result of too much current overheating the windings.
   In the high-voltage coils used in CRT TVs and LCD backlighting circuits, insulation
   between coil windings can break down and arc over, causing a short between a few
   windings but leaving most of them intact. That will change the coil’s inductance,
   making an inductance meter useful. To get any benefit from it however, you need to
   know what the correct inductance should be, and often there’s no way to ascertain
   that unless you have a known good coil with which to compare the suspect one. That,
   and the fact that coils don’t wear out and show gradually declining performance the
   way electrolytic capacitors do, accounts for the inductance meter being on the wish
   list, while the capacitance meter is a little higher up the chain of desire.


   Logic Analyzer
   An offshoot of the oscilloscope, a logic analyzer has lots of input channels but shows
   only whether signals are on or off. It is used to observe the timing relationships
   among multiple digital signal lines. Getting benefit from it requires knowledge of what
   those relationships should be, information rarely provided in the service manuals of
   consumer electronics devices. It is unlikely you’ll ever need one of these.


   SMT Rework Station
   Surface-mount technology, known as SMT, or sometimes SMD (surface-mount device),
   is today’s dominant style of componentry, because it makes for much smaller products
30   How to Diagnose and Fix Everything Electronic

     and also eliminates the need for drilling hundreds of very precise holes in the circuit
     board. SMT stuff is somewhat hard to work on, thanks to the size scale and the lack of
     holes to secure a part while you solder it. The pros use SMT rework stations, which are
     fancy soldering/desoldering stations with custom tips that fit various kinds of chips.
     SMT rework stations are expensive and not hobbyist material, at least so far.


     Spectrum Analyzer
     This is a special type of scope. Instead of plotting voltage versus time, a spec-an plots
     voltage versus frequency, letting you see how a signal occupies various parts of the
     frequency spectrum. Used extensively in design and testing of radio transmitters,
     spec-ans are expensive overkill for most service work, unless RF is your thing. Ham
     radio operators covet these costly instruments, but you won’t need one to fix normal
     consumer electronics devices. Besides, it is illegal to service transmitters in any
     way that could modify their spectral output unless you’re a licensed amateur radio
     operator working on ham gear or you hold a radiotelephone license authorizing you to
     work on such things.
Chapter                    3
   Danger, Danger! Staying Safe

   B   efore you start repairing electronics, get clear on one important fact: as soon as
       you crack open a product’s case, you have left the government-regulated, “I’ll sue
   you if this thing hurts me,” coddled, protected world of consumer electronics behind.
   Once the cover comes off, you are on your own, and you can get hurt or killed if you’re
   not careful! You’ve probably heard many times how dangerous CRT TV sets are to
   service, but don’t fool yourself into thinking that today’s gear is all that much safer.
   Even some battery-operated devices step up the voltage enough to zap the living crud
   out of you.
       That said, you can learn to navigate all kinds of repairs safely. Let’s look at a few
   ways you can get injured and how to avoid it, followed by the inverse: how you can
   damage the product you’re trying to service.



Electric Shock
   This is the most obvious hazard and the easiest to let happen. It might seem simple to
   avoid touching live connection points, but such contact happens all the time, because
   the insides of products are not designed for safety. Remember, you’re not supposed
   to be in there! You may find completely bare, unprotected spots harboring dangerous
   voltage, and a slip of the tool can be serious.
       Remove your wristwatch and jewelry before slipping your hand into a live
   electronic product. Yes, even a battery-operated one. Take off the wedding ring, too.
   They don’t call metal contact points terminals for nothing!
       In most devices, the electrical reference point called circuit ground is its metal
   chassis and/or metal shields. This is where old electrons go to die after having done
   their work, wending their way through the various components to get there. The trick
   is not to let them take you along for the ride! If you are in contact with the circuit
   ground point and also a point at a voltage higher than about 40 or 50 volts, you will
   get shocked. If your hands are moist, even lower voltages can zap you. The bodily
   harm from a shock arises from the current (number of electrons) passing through
   you, more than the voltage (their kick) itself. The higher the kick, though, the more

                                                                                         31
32   How to Diagnose and Fix Everything Electronic

     electrons it forces through your body’s resistance, which is why voltage matters. The
     path through your body is important as well, with the most dangerous being from
     hand to hand, because the current will flow across your chest and through your heart.
     That, of course, is one electrically regulated muscle whose rhythm you don’t want
     to interrupt. So, it’s prudent to keep your hand away from the circuit ground when
     taking measurements, just in case your other hand touches some significant voltage.
     In the old CRT TV service days, techs lived by the “one hand rule,” keeping one hand
     behind their backs while probing for signals in a powered set. Also, don’t service
     electronics while you’re barefoot or wearing socks; you’re more likely to be grounded,
     offering a path through your body for wayward electrons. Always wear shoes.
           Switching power supplies (see Chapter 14) have part of their circuitry directly
     connected to the AC line. As I mentioned in Chapter 2 on the section about isolation
     transformers, that’s a very dangerous thing, because lots of items around you in
     the room represent lovely ground points to which those unisolated electrons are
     just dying to go, and they don’t mind going through you to get there. Once again,
     never work on circuitry while it is directly connected to the AC line. If there’s no
     transformer between the AC line and the part of the circuit you wish to investigate,
     it’s directly connected. Unplug it from the line even before connecting your scope’s
     ground clip, because where you clip it may be at 120 volts or more, which will flow
     through the scope’s chassis on its way to the instrument’s ground connection, blowing
     fuses and possibly wrecking your scope.
           Lots of AC-operated products have exposed power supplies, with no protection
     at all over the fuse and other items directly connected to the AC line. Touching one
     of those parts is no different than sticking a screwdriver in a wall socket. It’s all
     too easy for the back of your hand to grant you a nasty surprise while your fingers
     and attention are aimed elsewhere. Even if the shock isn’t serious (which it could
     be), you’ll instinctively jerk your hand away and probably get cut on the machine’s
     chassis. When probing in a device with an exposed supply, place something
     nonconductive over the board when you’re not working on the supply itself. I like to
     use a piece of soft vinyl cut from the cover of a school notebook.
           Capacitors, especially large electrolytics, can store a serious amount of energy long
     after power has been removed. I’ve seen some that were still fully charged weeks later,
     though many circuits will bleed their energy off within seconds or minutes. The only
     way to be sure a cap is discharged is to discharge it yourself. Never do this by directly
     shorting its terminals! The current can be in the hundreds of amps, generating a huge
     spark and sometimes even welding your tool or wire to the terminals. Worse, that fast,
     furious flow can induce a gigantic current spike into the device’s circuitry, silently
     destroying transistors and chips. Instead, connect a 10-ohm resistor rated at a watt or
     two to a couple of clip leads, and clip them across the terminals to discharge the cap
     a little more slowly. Keep them connected for 20 seconds, and then remove one and
     measure across the cap with your DMM set to read DC voltage. It should read zero or
     close to it. If not, apply the resistor again until it does.
           Before discharging a cap, look at its voltage rating, because the voltage on it
     will always be less than the rating. If a cap is rated at 16 volts, it isn’t going to be
     dangerous. If it’s rated at 150 volts, watch out. Even with the low-voltage part, you
                                   Chapter 3      Danger, Danger! Staying Safe          33

    may want to discharge it before soldering or desoldering other components, to avoid
    causing momentary shorts that permit the cap’s stored energy to flow into places it
    doesn’t belong. Most of the time, low-voltage caps are in parts of circuits that cause
    the capacitors to discharge pretty rapidly once power is turned off, but not always.
         The capacitance value tells you how much energy the capacitor can store. A 0.1
    µf cap can’t store enough to cause you harm unless it’s charged to a high voltage, but
    when you have tens, hundreds or thousands of µf, the potential for an electrifying
    experience is considerable at the lower voltages you’re more likely to encounter.
         CRTs, especially in color TV sets, act like capacitors and have low enough leakage
    to store the high voltage applied to their anodes (the hole in the side with the rubber
    cap and the thick wire coming from it) for months. There isn’t much capacitance,
    thus not a lot of current, but the voltage is so high (anywhere from 25,000 to 50,000
    volts!) that what there is will go through you fast and hard enough to cause a large,
    sad family gathering about a week later. CRTs are going the way of the dodo, so you
    probably won’t work with them anyway. Just be very, very careful if you do. The
    terminals at the back of the tube carry some pretty high voltages too.
         The backlighting circuits of LCD monitors and TVs, along with much of the
    circuitry of plasma TVs, operate at high enough voltages to be treated with respect.
    It’s unwise to try to measure the output of a running backlighting circuit at the point
    where it connects to the fluorescent lamp tube unless you have a high-voltage probe
    made for that kind of work. Without one, you may get shocked from the voltage
    exceeding the breakdown rating of your probe, you’re likely to damage your DMM or
    scope, and the added load also may blow the backlighting circuit’s output transistors.
         Speaking of lamps, the high-pressure mercury vapor arc lamps used in video
    projectors are “struck,” or started, by putting around a kilovolt on them until they
    arc over, after which the voltage is reduced to about 100 volts. Keep clear of their
    connections during the striking period, and don’t try to measure that start-up voltage.



Physical Injury
    The outsides of products are carefully designed to be user-safe. Not so the insides!
    It’s easy to get sliced by component leads sticking up from solder joints, by the edges
    of metal shields, and even by plastic parts. Move deliberately and carefully; quickly
    shoving a hand into nooks and crannies leads to cuts, bleeding and cursing. That said,
    it still happens often enough that my years of tech work led me to coin the phrase,
    “No job is complete without a minor injury.”
          CD and DVD players and recorders (especially recorders) put out enough laser
    energy to harm your eyes, should you look into the beam. Video projectors use lamps
    so bright that you will seriously damage your vision by looking directly at them. The
    lamps and their housings get more than hot enough to burn you, and hot projection
    lamps are very fragile, so don’t bounce the unit or hit anything against it while it’s
    operating. An exploding lamp goes off like a little firecracker, oh-so-expensively
    showering you with fine glass particles and a little mercury, just for extra effect.
34   How to Diagnose and Fix Everything Electronic

          Speaking of eyes, yours will often be at rather close range to the work. Much of
     the time, you’ll be wearing magnifying lenses offering some protection from flying
     bits of wire or splattered molten solder. When the magnifiers aren’t in use, it’s a good
     idea to wear goggles, especially if you don’t wear glasses. Excess component leads
     clipped with diagonal cutters have an odd, almost magnetic tendency to head straight
     for your corneas at high speed. Solder smoke also likes to visit the area, and it can be
     pretty irritating.
          You can hurt your ears, too, particularly when working on audio amplifiers with
     speakers connected. Touching the wrong spot may produce a burst of hum or a squeal
     loud enough to do damage when your ears are close to the speakers. That sort of thing
     happens mostly with musical instrument amplifiers, because their speakers are right
     in your face when you work on them, and those amps pack quite a wallop. Even a 15-
     watt guitar amp can get painfully loud up close. Don’t think turning the volume knob
     down will protect you; there are plenty of places you can touch that will produce full
     power output regardless of the volume control’s position.
          Other opportunities for hearing damage involve using headphones to test
     malfunctioning audio gear. Even a little MP3 player with just a few milliwatts
     (thousandths of a watt) of output power can pump punishingly loud noises into your
     ears, particularly when ear buds that fit into the ear canals are used. If you must wear
     headphones to test a device, always use the over-the-ear type, and pull them back so
     they rest on the backs of your earlobes. That way, you can hear what’s going on, but
     unexpected loud noises won’t blast your eardrums.
          Breathing in solder smoke, contact cleaner spray and other service chemicals
     isn’t the healthiest activity. Keep your face away when spraying. When you must get
     close while soldering, holding your breath before the smoke rises can help you avoid
     inhalation.



Your Turn
     Sure, electronics can hurt you, but you can hurt the equipment too. Today’s devices
     are generally more delicate than those of past decades. It was pretty hard to damage
     a vacuum tube circuit with anything short of a dropped wrench hitting the glass.
     Today’s ultra-miniaturized circuitry is an entirely different slab of silicon. Here are
     some ways you can make a mess of your intended repair.


     Electrical Damage
     Working with powered circuits is essential in many repair jobs. You can’t scope signals
     when they’re off! Poking around in devices with power applied, though, presents
     great opportunity to cause a short, sending voltages to the wrong places and blowing
     semiconductors, many of which cannot withstand out-of-range voltages or currents for
     more than a fraction of a second. One of the easiest ways to trash a circuit is to press a
     probe against a solder pad on the board, only to have it slip off the curved surface when
     you look up at your test instrument, and wind up touching two pads at the same time.
                                 Chapter 3       Danger, Danger! Staying Safe             35

Any time you stick a probe on a solder pad, be very aware of this potential slip. At some
point, it’ll happen anyway, I promise you. Luckily, many times it causes no harm. Alas,
sometimes the results are disastrous. If you experience this oops-atronic event and
the circuit’s behavior suddenly changes, and toggling the power doesn’t restore it to its
previous state, assume you did some damage.
     Another common probing problem occurs when a scope probe is too large for
where you’re trying to poke it, and its ground ring, which is only a few millimeters
from the tip, touches a pad on the board, shorting it to ground. Again, sometimes you
get away with it, sometimes you don’t. In small-signal circuit stages, it’s more likely to
be harmless. In a power supply, well, you don’t want to do it, okay? It can be helpful
in tight circumstances to cut a small square of electrical tape and poke the end of the
probe through it, thus insulating the ground ring.
     When operating a unit with your bench power supply, there are several things
you can get wrong that will wreck the product. First and foremost, don’t connect
positive and negative backward! Nothing pops IC chips faster than reversed polarity.
Products subjected to it are often damaged beyond repair.
     Some devices, especially those intended for automotive use, have reverse-polarity
protection diodes across their DC power inputs. The diode, deliberately connected
backward with its anode to - and its cathode to +, doesn’t conduct as long as the power
is correctly applied. When polarity is reversed, the diode conducts, effectively shorting
out the power input and usually blowing the power supply’s fuse, protecting the
product’s sensitive transistors and chips from backward current. If the power supply
has a lot of current available, the diode may rapidly overheat and short, requiring its
replacement, but the rest of the unit should remain unharmed.
     Few battery-operated products have protection diodes. Reverse-polarity protection
is usually accomplished mechanically in the battery compartment by a recessed
terminal design that prevents the battery’s flat negative terminal from touching the
positive contact. The AC adapter jack probably isn’t protected either, because it’s
assumed you will use the adapter that came with the product.
     So, be very careful to connect your power supply the right way around, and never
hook it up while power is turned on, lest you even momentarily touch the terminals
with your clips reversed.
     Be sure you’ve set your power supply’s voltage correctly, too. Undervoltage rarely
causes damage, but overvoltage is likely to do so if it’s applied for more than a few
seconds. We’re not talking millivolts here; if you’re within half a volt or so, that’s usually
good enough. A decade ago, most items ran on unregulated linear-type adapters and
did the regulating internally, so they were fairly tolerant of having excessive voltage
coming in, and you were fine if you were within 2 or 3 volts. These days, more and
more products are using regulated, switching-type AC adapters with very steady voltage
outputs, so the gadgets expect a pretty accurate voltage.
     It’s possible to cause electrical damage when taking measurements, even if you
don’t slip with the probe. While scopes and meters have high impedance inputs that
won’t present any significant load to most circuits, sometimes you may be tempted
to connect resistors or capacitors across points to gauge the effects. That can be an
effective diagnostic technique in some cases, but it should be used with caution
because you can pull too much current through some other component and blow it.
36   How to Diagnose and Fix Everything Electronic

         Other times, you may want to connect a voltage to a point to see if it restores
     operation. That, too, can be useful, but it requires consideration of the correct voltage
     and polarity, the amount of current required, and exactly where that energy will go.
     Get one of those things wrong, and you could let some of that magic smoke out of the
     unit’s components, with predictable consequences.
         When your scope is set to AC coupling (see Chapter 6), it inserts a capacitor between
     the probe and the rest of the scope. After you probe a point with a DC voltage on it,
     that voltage remains on the capacitor and will be discharged back through the probe
     into the next point you touch. The amount of current is very, very small, but if you
     touch a connection to an especially sensitive IC chip or transistor that can’t handle the
     stored voltage, you could destroy the part in the short time it takes to discharge the
     scope’s capacitor. When using AC coupling, touch the probe to circuit ground between
     measurements to discharge the cap and prevent damage to delicate components.
         A static charge from rubbing your shoes across the carpet, or just the dry air of
     winter, can put hundreds of volts, or even a few thousand, on your fingertips and
     any tools you’re holding. If you think you could be charged, and especially any
     time you handle CMOS chips, MOSFET transistors, memory cards or other sensitive
     semiconductors, touch a grounded object first. The metal case of your bench power
     supply or analog scope should do the trick, as long as it’s plugged into a three-wire
     outlet. I don’t recommend using a digital scope as a discharge point because it has
     plenty of sensitive chips inside, and you sure don’t want to damage those!


     Physical Damage
     There are lots of ways you can break things when you’re inside a machine. One of
     the easiest is to tear a ribbon cable or snap off a critical part while disassembling the
     device. Some products pop apart easily but may have hidden risks. I once serviced
     a video projector cleverly designed to snap open without a single screw, but a tiny
     ribbon cable linked the top and bottom, and I was lucky that it popped out of its
     connector without being torn in half when I removed the top case. Had I pulled a little
     harder, I’d probably have done damage difficult to repair. As careful as I was trying to
     be, I still didn’t see that darned ribbon until it was too late.
          Small connectors of the sorts used on laptop motherboards and pocket camcorders
     can be torn from the circuit board. Today’s products are soldered by machine, and the
     soldering to connectors isn’t always the greatest, because they are a bit larger than
     the components, so they don’t get quite as warm during soldering. A little too much
     pressure when you disconnect the cable, and the connector can come right off the
     board. Depending on the size scale of its contacts, it may be impossible to resolder
     it. Most ribbon connectors have a release latch you must flip up or pull out before
     removing the ribbon. Always look for it before pulling on the cable. See Figures 3-1
     and 3-2.
          Your soldering iron, that magic instrument of thermo-healing, can also do a lot of
     damage, especially to plastic. The sides of the heating element can easily press against
     plastic cabinetry when you have to solder in tight places, melting it and ruining the
     unit’s cosmetics.
             Chapter 3    Danger, Danger! Staying Safe   37




Figure 3-1   Ribbon connector with flip-up latch




Figure 3-2   Ribbon connector with pull-out
latch
38   How to Diagnose and Fix Everything Electronic

         Finally, be careful where you press your fingers. Most circuitry is fairly hardy,
     but some components, including video heads, meters, speaker cones, microphone
     diaphragms, DLP projector color wheels and CD/DVD laser optical heads, just can’t
     stand much stress and will break if you push on them even with moderate force.



You Fixed It! Is It Safe?
     After repair, it’s your duty as a diligent device doctor to ensure the product is safe to
     use. One common error resulting in an unsafe repair job is neglecting to put everything
     back the way it was. If you have internal shields and covers or other items left over
     after you close the unit, you’ll need to open it back up and put them where they belong.
     Manufacturers don’t waste a single penny on unnecessary parts, so you know they’re
     important!
          It’s easy to touch wiring and melt insulation with the side of your soldering iron
     while you’re concentrating on soldering components. In units with lots of wires, it
     can happen, despite your best intention to be careful. You might not notice doing it if
     you’re focusing on the action at the tip, but the smoke and smell will alert you. Should
     you do this, fix the damage immediately; don’t wait until the repair is over. For one
     thing, you may have created a short or a lack of insulation that could cause damage or
     injury when power is applied. For another, you might forget later and close the unit
     up in that condition.
          Patching melted insulation can be as easy as remelting it to cover the wire, in the
     case of low-voltage, signal-carrying wires with only small melted spots. Or it might
     require cutting, splicing and heat-shrink tubing if the wire handles serious voltage,
     or if the damage is too great. Remember, electrical tape will come off after awhile, so
     never depend on it for long-term safety.
          If you’ve replaced power-handling components like output transistors or voltage
     regulators, be sure to test the unit for proper operation and excess heat. Older stereo
     amps and receivers, for instance, sometimes require bias adjustments when the output
     transistors are changed. If you don’t set the bias correctly, the unit will work for awhile,
     but it may overheat badly. Let it run on the bench for a few hours at normal listening
     volume and see how hot it gets. Be sure what you’ve fixed is really working properly
     before you close it up.
          When the product has an AC cord, take a look at it and run your hand along its
     entire length, checking for cuts. Naturally, you want to live, so unplug it before doing
     this! You’ll be amazed at how many frayed, cut and pet-chewed cords are out there.
     Replace the cord or repair it as seems appropriate for its condition, paying extra
     attention to a good, clean job with proper insulation. If the damage is only to one
     wire, it’s easier to fix than if both sides are involved, because at least the two wires
     can’t short to each other. With a damaged AC cord, I like to use both electrical tape
     and heat-shrink tubing over it.
Chapter                 4
 I Fix, Therefore I Am: The
 Philosophy of Troubleshooting

 I  magine if your doctor saw you as a collection of organs, nerves and bones, never
    considering the synergistic result of their working together, supplying each other
 with the chemicals and signals necessary for life. No organ could survive on its
 own, but together they make a living, breathing, occasionally snoring you! Now
 consider how tough it’d be to solve a murder case without considering the motives,
 personalities and circumstances of the victim and all potential suspects. The knife is
 right there next to the body, but anybody on earth could have done the crime. Why
 was the victim killed? Who knew him? Who might have wanted him dead?
      Troubleshooting, which involves skills somewhat like those of doctors and
 detectives, is a lot like that. You can think of an electronic device as a bunch of
 transistors, chips and capacitors stuffed into a box, and sometimes that’s enough
 to find simple failures. Taking such a myopic view, though, limits you to being a
 mediocre technician, one who will be stumped when the problem isn’t obvious. To be
 a top-notch tech requires consideration of the bigger picture. Who made this product,
 and what were the design goals? How is it supposed to work? How do various sections
 interact, and what is the likely result of a failure of one area on another?
      Machines are systems. Being built by humans, they naturally reflect our
 biological origins, with cameras for eyes, microphones for ears, speakers for larynxes
 and microprocessors for brains. Even the names of many parts sound like us:
 tape recorders, hard drives, and optical disc players have heads, turntables have
 arms, chips have legs and picture tubes have necks. Some products even exhibit
 personalities, or at least it feels that way to us. Their features and quirks can be
 irritating, humorous or soothing. Their failures are much like our own, too, with
 symptoms that may be far removed from what’s causing them, thanks to some
 obscure interaction that nobody, not even the circuit’s designer, could have foreseen.
      The more you come to understand how devices work at the macro level, the
 more sense their problems will make. The more you can consider products as metal
 and silicon expressions of human thinking, the better sleuthing skills you will attain.
 Before we get to the nitty-gritty of transistors, current flow and signals, let’s put on

                                                                                      39
40   How to Diagnose and Fix Everything Electronic

     our philosophers’ hats and become the Socrates of circuitry, the Erasmus of electrons.
     Let’s look at why products work and why they don’t, and how to avoid some of the
     common pitfalls developing techs encounter. Let’s become one with the machines.



Why Things Work in the First Place
     When you get a few thousand parts together and apply power to them, they can
     interact in many ways. The designing engineer had one particular way in mind, but
     that doesn’t mean the confounded conglomeration of components will cooperate!
          Analog circuitry has a wider range of variation in its behaviors than does digital,
     but even today’s all-digital gear can be surprisingly inconsistent. I’ve witnessed two
     identical laptop computers running exactly the same software, with exactly the same
     settings, but drawing significantly different amounts of current from their power
     supplies. I’ve also seen all kinds of minor variations in color quality between identical
     digital still and video cameras. I remember a ham radio transceiver whose digital
     control system exhibited a bizarre, obscure behavior in its memory storage operation
     that no other radio of that model was reported to have, and I never found any bad
     parts that might explain the symptom. I finally had to modify the radio to get it to
     work like all the others.
          Sure, you string a few gates together and you will be able to predict their every
     state. Get a few thousand or more going, run them millions of times per second, and
     mysterious behaviors may start to crop up.
          It’s useful to think of all circuitry as a collection of resistors impeding the passage
     of current from the power supply terminal to circuit ground. As the current trickles
     through them, it is used to do work, be it switching the gates in a microprocessor,
     generating laser light for a disc player, or spinning the disc. Electrons, though,
     are little devils that will go anywhere they can. If there’s a path, they’ll find it.
     Malfunctions can be considered either as paths that shouldn’t be there or a lack of
     paths that should.
          In essence, when machines work properly, it’s because they have no choice. The
     designer has carefully considered all the possible paths and correctly engineered the
     circuit to keep those pesky electrons moving along only where and when they should,
     locking out all possible behaviors except the desired one. When choice arises, through
     failing components, user-inflicted damage or design errors, the electrons go on a spree
     like college students at spring break, and the unit lands on your workbench.



Products as Art
     A machine is an extension of its designer much as a concerto is an extension of its
     composer. Beethoven sounds like Beethoven, and never like Rachmaninoff, because
     Ludwig’s bag of tricks and way of thinking were uniquely his, right? It’s much the
     same with products. In this case, however, they tend to have unifying characteristics
     more reflective of their manufacturing companies than of a specific person. Still,
   Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting              41

   I suspect that an individual engineer’s or manager’s viewpoints and preferences set
   the standard, good or bad, which lives on in a company’s product line long after that
   employee’s retirement.
       Understanding that companies have divergent design philosophies and quirks
   may help your repair work, because you can keep an eye on issues that tend to crop
   up in different manufacturers’ machines. You may notice that digital cameras from
   one maker have a high rate of imaging chip failures, so you’ll go looking for that
   instead of some other related problem when a troublesome case hits your bench. Or
   perhaps you’ve found that tape-type camcorders from a particular company often
   have mechanical loading problems because that manufacturer uses loading arms and
   other metal structures in the tape transport that are too thin, so they bend.
       When you’ve fixed enough products, you’ll begin to recognize what company made
   a machine just by looking at its circuit board or mechanical sections. The layouts, the
   styles of capacitors, the connectors, and even the overall look of the copper traces on
   a board are different and consistent enough to be dead giveaways.



If It Only Had a Brain
   Continuing our anatomical analogy, yesterday’s tech product was like a zombie. Perhaps
   it had an ear (microphone), some memory (recording tape) and a mouth (speaker).
   Each system did its simple job, with support from a stomach (power supply) and some
   muscles (motors, amplifiers).
         What was missing was a brain. Today’s gear is cranium-heavy, laden with computing
   power. Gone are simple mechanical linkages to control sequencing and movement of
   mechanisms. Instead, individual actuators move parts in a sequence determined by
   software, positional information gets fed back to the microprocessor, and malfunctions
   might originate in the mechanics, the sensors, the software, or some subtle interaction
   of those elements. No longer are there potentiometers (variable resistors) to set volume
   or brightness; buttons signal the brain to change the parameters. Heck, most gadgets
   today don’t even have “hard” on/off switches that actually disconnect power from
   the circuitry. Instead, the power button does nothing more than send a signal to the
   microprocessor, requesting it to energize or shut down the product’s circuitry.
         In addition to the brain, many modern products have nervous systems consisting
   of intermediary chips and transistors to decode the micro’s commands and fan
   them out to the various muscles, and organs doing the actual work. Failures in these
   areas can be tough to trace, because their incoming signals from the computer chip
   are dependent on tricky timing relationships between various signal lines. This
   is a profound shift from the old way of building devices, and it adds new layers of
   complication to repair work. Is the circuit not working due to its own malfunction, or
   is it playing dead because the micro didn’t wake it up?
         Today’s machines are complete electrono-beings with pretty complex heads on
   their shoulders. Some offer updatable software, while many have the coding hardwired
   into their chips. Which would you like to be today: surgeon or psychiatrist?
42   How to Diagnose and Fix Everything Electronic


The Good, the Bad and the Sloppy
     It’s easy for an experienced tech to tell when a repair attempt has been made by an
     unqualified person. The screws will be stripped, or there will be poorly soldered joints
     with splashes of dripped solder lying across pads on the board. Wires may be spliced
     with no solder and, perhaps, covered in cellophane tape, if at all. Adjustments will be
     turned, insulation melted, and so on. In a word: sloppiness.
          That might sound exaggerated, but I used to run into it a lot when I worked in
     repair facilities. Most shops have policies of refusing to work on items mangled by
     amateurs, so discovery of obvious, inept tampering was followed by a phone call to
     the item’s owner, who would stubbornly insist that the unit had never been apart and
     had simply quit working. Um, right, Sony used Scotch tape to join unsoldered wires.
     Sure, buddy. I remember one incident in which I refused to repair a badly damaged
     and obviously tampered-with shortwave radio. The owner was so angry that he called
     my boss and tried to have me fired! The boss took one look inside the set, clapped
     me on the back, laughed, and told the guy to come pick up his ruined radio and go
     away. Don’tcha wish all bosses were that great? The key to performing a proper,
     professional-quality repair job is meticulous attention to detail. Think of yourself as
     a surgeon, for that’s exactly what you are. You are about to open up the body of this
     mechanical “organism” and attempt to right its ills. As the medical saying goes, “First,
     do no harm.” Now and then, repair jobs go awry and machines get ruined—it happens
     even to the best techs, though rarely—but your aim is to get in and back out as cleanly
     as possible. In Chapters 9 through 13, we’ll explore the steps and techniques required
     for proper disassembly, repair and reassembly.



Mistakes Beginners Make
     Beyond sloppy work, beginners tend to make a few conceptual errors, leading to lots
     of lost time, internal damage to products, and failure to find and fix the problem. Here
     are some common quagmires to avoid.


     Adjusting to Cover the Real Trouble
     Analog devices often have adjustments to keep their circuit stages producing signals
     with the characteristics required for the other stages to do their jobs properly. TVs and
     radios are full of trimpots (variable resistors), trimcaps (variable capacitors) and tunable
     coils, and their interactions can be quite complex. With today’s overwhelmingly digital
     circuits, adjustments are much less common. Many are performed in software with
     special programming devices to which you won’t have access, but some good-old-
     fashioned screwdriver-adjustable parts still exist. Power supplies usually have voltage
     adjustments, for instance, and earlier-generation CD players were loaded with servo
     adjustments to keep the laser beam properly focused and centered on the track. Even
     a digital media receiver may have tunable stages in its radio sections.
          It can be very tempting to twiddle with adjustments in the hope that the device will
     return to normal operation. While it’s true that circuits do go out of alignment—if they
Chapter 4      I Fix, Therefore I Am: The Philosophy of Troubleshooting                  43

didn’t, the controls wouldn’t be there in the first place—that is a gradual process. It never
causes drastic changes in performance. If the unit suddenly won’t do something it did
fine the day before, it’s not out of adjustment, it’s broken. Messing with the adjustments
will only get you into trouble later on when you find the real problem, and now the
machine really is way out of alignment, because you made it that way. Leave those
internal controls alone! Turn them only when you’re certain everything else is working,
and then only if you know precisely what they do and have a sure way to put them
back the way they were, just in case you’re wrong. Marking the positions of trimpots
and trimcaps with a felt-tip marker before you turn them can help, but it’s no guarantee
you will be able to reset a control exactly to its original position. There’s too much
mechanical play in them for that technique to be reliable. In some cases, close is good
enough. In others, slight misadjustments can seriously degrade circuit performance.
     I once worked on a pair of infrared cordless headphones with a weak, distorted
right channel. After some testing, it was clear that the transmitter was the culprit, and
its oscillator for that channel had drifted off frequency. A quick adjustment and, sure
enough, the headphones worked fine for a little while. Then the symptom returned.
The real problem: a voltage regulator that was drifting with temperature. Luckily,
readjusting the oscillator was easy after the new part was installed. When multiple
adjustments have been made, it can be exceedingly difficult to get them back in
proper balance with each other.


Making the Data Fit the Theory
Most techs have been guilty of this at some time. In my early years, mea culpa, that’s
for sure. You look at the symptoms, and they seem to point to a clear diagnosis—all
except for one. You fixate on those that make sense, convince yourself that they add
up, and do your best to ignore that anomaly, hoping it’s not significant. Trust me, it
is, and you are about to embark on a long, frustrating hunting expedition leading to a
dreary dead end. Always keep this in mind: If a puzzle won’t fit together, there’s a piece
missing! There’s something you don’t know, and that is what you should be chasing.
Often, the anomaly you’re pushing aside is the real clue, and overlooking it is the
worst mistake you can make. Many maddening hours later, when you finally do solve
the mystery, you’ll think to yourself, “Why didn’t I consider how that odd symptom
might be the key to the whole thing? It was right in front of me from the start!” Ah,
hindsight…. Nobody needs glasses for that.


Going Around in Circles
Sometimes you think you’ve found the problem, but trying to solve it creates new
problems, so you go after those. Those lead to still more odd circuit behavior, so
off you go, around and around until you’re right back where you started. When
addressing symptoms creates more symptoms, take it as a strong hint that you are
on the wrong track. It’s incredibly rare for multiple, unrelated breakdowns to occur.
Almost always, there is one root cause of all the strangeness, and it’ll make total
sense once you find it. “Oh, the power supply voltage was too low, and that’s why
44   How to Diagnose and Fix Everything Electronic

     the focus wouldn’t lock and the sled motor wouldn’t make the laser head go looking
     for the track.” If you’re lucky, you’ll have discovered that before you’ve spent hours
     fiddling with the limit switches and the control circuitry, tracing signals back to the
     microprocessor. Again, if the puzzle won’t fit together, find that missing piece!



That’s How It Goes
     As with illness in the human body, just about anything can go wrong with an electronic
     device. Problems range from the obvious to the obscure; I’ve fixed machines in 5 minutes,
     and I’ve run across some oddball cases for which a diagnosis of demonic possession
     seemed appropriate! These digital days, circuitry is much more reliable than in the
     old analog age, yet modern gear often has a much shorter life span. How can both of
     those statements be true?
          Today’s products are of tremendously greater complexity, with lots of components,
     interconnections and interactions, so there’s more to go wrong. Unlike the hand-
     soldered boards filled with a wide variety of component types we used to have, today’s
     small-signal boards, with their rows of surface-mounted, machine-soldered chips,
     don’t fail that often. But with so much more going on, they include complicated power
     supplies and a multitude of connectors and ribbon cables. Plus, some parts work much
     harder than they used to and wear out or fail catastrophically from the stress. And
     thanks to the rapid pace of technological change, the competition to produce products
     at bare-bones prices and the high cost of repair versus replacement, extended longevity
     is not the design goal it once was. Manufacturers figure you’ll want to buy a new,
     more advanced gadget in a couple of years anyway. Contrary to popular myth, nobody
     deliberately builds things to break. They don’t have to; keeping affordable products
     working for long periods is tough enough. Keeping expensive items functioning isn’t
     easy either! Laptop computers, some of the costliest gadgets around, are also some of
     the most failure-prone, because they’re very complex and densely packed, and they
     produce plenty of heat.
          It may seem like electronic breakdowns are pretty random. Some part blows for
     reasons no one can fathom, and the unit just quits. That does happen, but it’s not
     common. Oh, sure, when you make millions of chips, capacitors and transistors, a
     small number of flawed ones will slip through quality control, no matter how much
     testing you do. It’s a tiny percentage, though. Much more often, products fail in a
     somewhat predictable pattern, with a cascading series of events stemming from well-
     recognized weaknesses inherent in certain types of components and construction
     techniques. In other words, nothing is perfect! Let’s look at the factors behind most
     product failures.


     Infant Mortality
     This rather unpleasant term refers to that percentage of units destined to stop working
     very soon after being put into service. Imperfect solder joints, molecular-level flaws in
     semiconductors and design errors cause most of these. While many products are tested
Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting                45

after construction, cost and time constraints prohibit extensive “burning in” of all but
very expensive machines. Typical infant mortality cases crop up within a week or two
of purchase and land in a warranty repair center after being returned for exchange. So,
you may never see one unless you bought something from halfway around the world,
and it’s not worth the expense and trouble to return it. Or perhaps the seller refuses to
accept it back, and you get stuck with a brand-new, dead device you want to resurrect.


Mechanical Wear
By far, moving parts break down more often than do electronic components. Hard
drives, VCR and camcorder mechanisms, disc trays, laser head sleds and disc-spinning
motors are all huge sources of trouble.
    Bearings wear out, lubrication dries up, rubber belts stretch, leaf switches (internal
position-sensing switches) bend, nylon gears split, pet hairs bind motor shafts, and
good old wear and tear grind down just about anything that rubs or presses against
anything else. If a device has moving parts and it turns on but doesn’t work properly,
look at those first before assuming the electronics behind them are faulty. For every
transistor you will change, you’ll fix five mechanical problems.


Connections
Connections are also mechanical, and they go bad very, very often. Suspect any
connection in which contacts are pressed against each other without being soldered.
That category includes switches, relays, plugs, sockets, and ribbon cables and connectors.
    The primary culprit is corrosion of the contacts, caused by age and sometimes, in
the case of switches and relays, sparking when the contacts are opened and closed.
Also, a type of lubricating grease used by some manufacturers on leaf switches tends
to dry out over time and become an effective insulator. If the contact points on a leaf
switch are black, it’s a good bet they are coated with this stuff and are not passing any
current when the switch closes. See Figure 4-1.




                   Figure 4-1     Leaf switch
46   How to Diagnose and Fix Everything Electronic

           A particularly nasty type of bad connection occurs in multilayer printed circuit
     boards. At one time, a dual-layer board, with traces on both sides, was an exotic
     construct employed only in the highest-end products. Today, dual-layer boards are
     pretty much standard in larger, simpler devices, while smaller, more complex gadgets
     may utilize as many as six layers!
           The problems crop up in the connections between layers. Those connections are
     constructed differently by the various manufacturers. The best, most reliable style is
     with plated-through holes, in which copper plating joins the layers. As boards have
     shrunk, plated-through construction has gotten more difficult, resulting in a newer
     technique that is, alas, far less reliable: holes filled with conductive glue. This type of
     interconnect is recognizable by a raised bump at the connection point that looks like,
     well, a blob of glue (see the translucent glue over the holes in Figure 4-2). Conductive
     glue can fail from flexure of the board, excessive current and repeated temperature
     swings. Repairing bad glue interconnects is hard, too. I always cringe when I see those
     little blobs.


     Solder Joints
     Though they’re supposed to be molecularly bonded and should last indefinitely,
     solder joints frequently fail and develop resistance, impeding or stopping the current.
     When it happens in small-signal, cool-running circuitry, it’s usually the fault of a
     flaw in the manufacturing process, even if it takes years to show up. Heat-generating
     components like output transistors, voltage regulators and video processing chips on
     computer motherboards can run hot enough to degrade their solder joints gradually
     without getting up to a temperature high enough to actually melt them. Over time,
     the damage gets done and the joints become resistive or intermittent.




                      Figure 4-2     Conductive glue interconnects
Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting                 47

     Many bad solder joints are visually identifiable by their dull, mottled or cracked
appearance. Now and then, though, you’ll find one that looks perfect but still doesn’t
work, because the incomplete molecular bonding lies beneath the surface. Bonding
may be poor due to corrosion on the lead or pad of the soldered components; solder
just won’t flow into corroded or oxidized metal. When you go to resolder it, you’ll have
problems getting a good joint unless you scrape things clean first, after removing the
old solder.


Heat Stress
Heat is the enemy of electronics. It’s not an issue with most pocket-sized gadgets, but
larger items like video projectors, TVs and audio amplifiers often fail from excessive
heating. So do backlight inverters (the circuits that light the fluorescent lamps behind
LCD screens) and computer motherboards. Power supplies create a fair amount of
heat and are especially prone to dying from it.
    Overheating from excessive current due to a shorted component can quickly
destroy semiconductors and resistors, but normal heat generated by using a properly
functioning product can also gradually degrade electrolytic capacitors, those big ones
used as power supply filtering elements, until they lose most of their capacitance.


Electrical Stress
Running a device on too high a voltage can damage it in many ways. The unit’s
voltage regulator may overheat from dissipating all the extra power, especially if it’s
a linear regulator. Electrolytic capacitors can short out from being run too close to, or
over, their voltage limits. Semiconductors with inherent voltage requirements may die
very quickly.
     Overvoltage can be applied by using the wrong AC adapter, a malfunctioning
adapter, a bad voltage regulator, or using alkaline batteries in a device made for
operation only with nickel-metal hydride (NiMH) rechargeable cells. Those cells
produce 1.2 volts each, compared to the 1.5 volts of alkalines. So, with four cells, you
get 6 volts with the alks, compared to the 5 volts the device expects. Most circuits can
handle that, but some can’t.
     Believe it or not, a few products can be damaged by too little voltage. Devices with
switching power supplies or regulators compensate for the lower voltage by pushing
more current through their transformers with wider pulses, to keep the output voltage
at its required level. That can cause overheating of the rectifiers and other parts
converting the pulses back to regulated DC.
     The ultimate electrical stress is a lightning strike. A direct strike, as may occur to
a TV or radio with an outdoor antenna that gets zapped, or from a hit to the AC line,
will probably result in complete destruction of the product. Now and then, only one
section is destroyed and the rest survives, but don’t bet on it. Lightning cases tend to
be write-offs; you don’t even want their remains in your stack of old boards, lest their
surviving parts have internal damage limiting their life spans.
48   How to Diagnose and Fix Everything Electronic

          Power surges, in which the AC line’s voltage rises to high levels only momentarily,
     can do plenty of damage. Such surges are sometimes the result of utility company
     errors, but more often lightning has struck nearby and induced the surge without
     actually hitting the line, or it has hit the line far away. Often, the power supply
     section of the product is badly damaged but the rest of the unit is unharmed.
          When too much current passes through components, they overheat and can
     burn out, sometimes literally. Resistors get reduced to little shards of carbon, and
     transistors can exhibit cracks in their plastic cases. The innards, of course, are wiped
     out. This kind of stress rarely occurs from outside, because you can’t force current
     through a circuit; that takes voltage. When overcurrent occurs, it’s because some
     other component is shorting to ground, pulling excessive current through whatever is
     connected in series with it.
          Nothing kills solid-state circuitry quite as fast as reversed polarity. Many
     semiconductors, and especially IC chips, can’t handle current going the wrong way
     for more than a fraction of a second.
          Batteries can be installed backward. Back when 9-volt batteries were the power
     source of choice for pocket gadgets, all it took was to touch the battery to the clip with
     the male and female contacts the wrong way around and the power switch turned
     on. Now that AAA cells and proprietary rechargeable batteries run our diminutive
     delights, that kind of error occurs less often, because it’s routine for designers to shape
     battery compartments to prevent reversed contacts from touching, but it still happens
     on occasion.
          By far, the most frequent cause of reversed polarity is an attempt to power a
     device from the wrong AC adapter. Today, most AC adapters connect positive to
     the center of their coaxial DC power plugs and negative to the outside, so that an
     automotive cigarette lighter adapter made for the same gadget doesn’t present the
     risk of having positive come in contact with the metal car body, which would cause a
     short and blow the car’s fuse. At one time, though, many adapters had negative on the
     center instead, and a few still do on items like answering machines, which will never
     be used in cars. Even from the same manufacturer, both schemes may be employed
     on their various products.
          The train wreck occurs when the user plugs in the wrong adapter, and it happens
     to have the plug wired opposite to what the device wants. Damage may be limited
     only to a few parts in the power supply section, or it can be extreme, taking out
     critical components like microprocessors and display drivers.
          Not all electrical stress is caused by external factors or random component failures.
     Sometimes design errors are inherent in a product, and their resulting malfunctions
     don’t start showing up until many units are in the field for awhile. When a manufacturer
     begins getting lots of warranty repair claims for the same failure, the alarm bells go off,
     and a respectable company issues an ECO, or engineering change order, to amend the
     design. Units brought in for repair get updated parts, correcting the problem. A really
     diligent manufacturer will extend free ECO repairs beyond the warranty period if it’s
     clear that the design fault is bad enough to render all or most of the machines in the
     field inoperative, or if any danger to the user could be involved.
Chapter 4      I Fix, Therefore I Am: The Philosophy of Troubleshooting                   49

     At least that’s how it’s supposed to work. Sometimes companies don’t want
to spend the money to fix their mistakes, so they simply deny the problem. Or, if
only some machines exhibit the symptom, they’re treated as random failures, even
though they’re not. Perhaps it takes a certain kind of use or sequence of operations
for the issue to become evident, and the manufacturer genuinely believes the design
is sound. And some units aren’t used often enough to have experienced the failure,
though they will eventually, masking its ultimate ubiquity.
     Any of these situations can result in your working on a product with a problem
that will recur, perhaps months later, after you’ve properly solved it. If the thing keeps
coming back with the same issue, suspect a defective design.


Physical Stress
Chips, transistors, resistors and capacitors can take the physical shock of being dropped,
at least most of the time. Many other parts can’t, though. Circuit boards can crack,
especially near the edges and around screw holes and other support points. Larger
parts, with their greater mass, can break the board areas around them. That happens
often with transformers and big capacitors. On a single or dual-layer board, you may be
able to bridge foil traces over the crack with small pieces of wire and a little solder if the
traces are not too small. With a multilayer board, you may as well toss the machine on
the parts pile, because it’s toast.
     LCDs, fluorescent tubes and other glass displays rarely survive a drop to a hard
surface. The very thin, long fluorescent lamps inside laptop screens are particularly
vulnerable to breakage. If you run across a laptop with no backlight, don’t be too
surprised if it got dropped and the lamps are broken inside the screen. I’ve seen that
happen with no damage to the LCD itself being evident.
     If you leave carbon-zinc or alkaline batteries installed long enough, they will leak.
Not maybe, not sometimes, they will. Devices like digital cameras take a fair amount
of current and get their batteries changed often, but those with low current demand,
such as digital clocks and some kids’ toys, may have the same batteries left in them
for years. Remote controls are prime candidates for battery leakage damage, because
most people install the cheap, low-quality batteries that come with them and never
change them; their very low usage ensures those junky cells will be in there until
they rot.
     Once the goo comes out, you’re in for a lot of work cleaning up the mess. They
don’t call them alkaline batteries for nothing! The electrolyte is quite corrosive and
will eat the unit’s battery springs and contacts. If the stuff gets inside and onto the
circuit board, that’s where the bigger calamity goes down. Copper traces will be eaten
through, solder pads corroded, and those pesky circuit board layer interconnects will
stop working. No shop will try to repair such damage, but you might want to give it a
go if the device is expensive or hard to replace.
     People sit on their phones, PDAs and digital cameras fairly often, resulting in
cracked LCDs, broken circuit boards and flattened metal cases shorting components to
circuit ground. It’s easy to bend a case back to an approximation of its original shape,
but the mess inside may not be worth the trouble.
50   How to Diagnose and Fix Everything Electronic

          Liquid and electronics don’t mix, yet people try to combine them all the time,
     spilling coffee, wine and soft drinks into their laptops, and dropping their cameras and
     phones into the ocean and swimming pools. Good luck trying to save such items. Now
     and then you can wash them out with distilled water, let them dry for a long time, and
     wind up with a functional product. Most of the time, and especially with salt-water
     intrusion, it’s a total loss.
          Just being near salt water will destroy electronics after awhile. Two-way radios,
     navigation systems, stereos and TVs kept on a boat or even in a seaside apartment get
     badly corroded inside, with rusted chassis; dull, damaged solder joints; and connectors
     that don’t pass current. Very often you’ll see crusty green crud all over everything.
          Speaking of the ocean, the beach is a prime killing ground for cameras. Most
     digital cameras feature lenses that extend when the camera is powered on. Any sand
     in the cracks between lens sections will work its way into the extending mechanism
     and freeze that baby up, and it is very hard to get all the grit out. In a typical case, the
     camera is dropped lens-first into the sand, and a great deal of it gets inside. I’ve taken
     a few apart and disassembled the lens assemblies, cleaned half a beach out of them,
     and still had little luck restoring their operation. There’s always a few grains of sand
     somewhere deep in those nylon gears, where you can’t find them, and even one grain
     can stop the whole works.


     The Great Capacitor Scandal
     Around 1990, a worker at an Asian capacitor plant stole the company’s formula, fled
     to Taiwan, and opened his own manufacturing plant, cranking out millions of surface-
     mount electrolytic capacitors that found their way into countless consumer products
     from the major makers we all know and love. A few other Taiwanese capacitor makers
     copied the formula too.
          Alas, that formula contained an error that caused the electrolyte in those caps to
     break down and release hydrogen. Over a few years, the caps swelled and burst their
     rubber seals, releasing corrosive electrolyte onto the products’ circuit boards, severely
     damaging them and ruining the units.
          This ugly little secret didn’t become well known for quite awhile, until long after
     the warranty periods were expired. Billions of dollars’ worth of camcorders and other
     costly small products were lost, all at their owners’ expense. Any attempt at having
     repairs made was met with a diagnosis of “liquid damage—unrepairable.” The disaster
     was so pervasive, and took long enough to show up, that many companies insisted
     the failures were random and have never to this day admitted any liability for the
     lost value.
          More recently, similar electrolyte problems have continued to plague computer
     motherboards and the power supplies of various products, affecting even their full-sized
     capacitors with leads. Caps are dying after just a year or two of use. The higher heat of
     lead-free soldering also may be contributing to early capacitor failure.
          Lawsuits have been filed, and remedial action has been taken by some manufacturers
     to purge their product lines of the offending parts. Still, it is highly likely you will
   Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting               51

   run into bulging capacitors in your repair work, perhaps more than any other single
   cause of failure. Even when they’re not bulging, the caps may lose their ability to store
   energy, showing almost no capacitance on a capacitance meter.



History Lessons
   A good doctor understands the value of taking the patient’s history before performing
   an examination. Knowing the factors leading up to the complaint can be very valuable
   in assessing the cause. How old are you? Do you smoke? Drink? Have a family history
   of this illness? What were you doing when symptoms appeared?
        If you have access to a machine’s history, it can provide the same kinds of helpful
   hints, often leading you to a preliminary diagnosis before you even try to turn it on.
   Here are some factors worth considering before the initial evaluation:

    •	 Who made it? As discussed earlier, products from specific companies can have
       frequently occurring problems due to design and manufacturing philosophy.
       Becoming familiar with those differences may help guide you to likely issues,
       especially if you’ve seen the problem before in another unit, even of a different
       model, from the same maker.
       It pays to check the Internet for reports of similar troubles with the same model
       product. You may save many hours of wheel reinvention by discovering that
       others are complaining about the same failure. You might find the cure, too.
    •	 How old is it? If made before the 1990s, it shouldn’t have the leaking capacitor
       problem. It could have a lot of wear, though, with breakdowns related to plenty of
       hours of use. If it was made in the ’90s or more recently, those caps are a prime
       suspect.
    •	 Has it been abused? Dropped? Dunked? Spilled into? Sat on? Left on the dashboard
       of a car in the summer? Used at the beach? Had batteries in it for months or
       years? Had a disc or tape stuck in it, and somebody tried to tear it out? Been in
       a thunderstorm? Through the washing machine? Kept on a boat? Played with by
       kids? Cranked up at maximum volume in a club for long periods?
       Each of these conditions can lead you down the diagnosis path. A stereo amplifier
       used gently at home by 70-year-olds is likely to have a very different failure than
       one cranked up to high volume levels in a club, or one run 40 hours a week in a
       restaurant for 10 years.
    •	 What was it doing when it failed? While gadgets sometimes quit while in operation,
       many stop working when sitting idle, and the problem isn’t discovered until
       the next time someone tries to use the product. This is particularly true of AC-
       powered machines that, like most things today, have remote controls. To sense
       and interpret the turn-on signal from the remote, at least some of the circuitry has
       to be kept active at all times. VCRs, DVRs, DVD players and TVs are never truly
       turned off; some power always flows. A power surge, a quick spike, or perhaps
       just age or—as always—bad capacitors can kill the standby supply, resulting in
       complete loss of operation.
52   How to Diagnose and Fix Everything Electronic

         If the device did crash while being used, it’s very helpful to know precisely what
         operation was being carried out when it quit. If a laptop’s backlight went dead
         while the screen was being tilted, for example, that’s a good indication of a broken
         internal cable, rather than a blown transistor in the backlight inverter.
      •	 Did it do something weird shortly before quitting? Many failing circuits exhibit
         odd operation for anywhere from minutes to seconds before they shut down
         altogether. That peculiar behavior can contain clues to the cause of their demise.
         In fact, it usually does, and it may hold the only hints you have in cases of total
         loss of function.
      •	 Was it sudden or gradual? Some causes of failure, such as drifting alignment, dirty
         or worn mechanisms, and leaking or drying electrolytic capacitors, may manifest
         gradually over time. Bad caps on computer motherboards are a great example of
         this, as they cause the machine to get less and less stable, with more and more
         frequent crashes, until bootup is no longer possible.

          Parts don’t blow gradually, though. While it’s possible in rare cases for components,
     and especially transistors, to exhibit intermittent bad behavior, a truly blown (open-
     circuited) component goes suddenly and permanently, frequently shorting first and then
     opening a moment later from the heat of all the current passing through its short. So,
     if the symptoms appeared gradually, it’s a safe bet that the problem is not blown parts.



Stick Out Your USB Port and Say “Ahhh”:
Initial Evaluation
     Before you take a unit apart, examine it externally and try to form a hypothesis
     describing its failure. The most potent paintbrush in the diagnostic art is simple logic.
     Your first brushstroke should be to reduce variables and eliminate as many areas of
     the circuitry from consideration as you can. Instead of chasing what might be wrong,
     first focus on what the problem can’t be. By doing so, you’ll sidestep hours of signal
     tracing and frustration. Before you open the unit, give some thought to these issues.

     It’s dead, Jim! “Dead” is a word many people use when something doesn’t work,
     but often it’s incorrectly applied. If anything at all happens when you apply power, the
     thing isn’t dead! A lit LED, a display with something—even something scrambled and
     meaningless—on it, a hum, a hiss, some warmth, or any activity whatsoever, indicates
     that the circuitry is getting some power from the power supply, at least. “Dead” means
     dead. Zip, nada, nothing, stone cold. If you do see signs of life, some power supply
     voltage could still be missing or far from its correct value, but the supply is less likely
     to be the problem. In a product with a switching supply, you can assume that the
     chopper transistor is good, as are the fuse and the bridge rectifier. You can’t be sure
     the supply has no other problem like bad capacitors or poor voltage regulation.
          If the device is totally dead, check the fuse. All AC-powered products have
     fuses, and so do most battery-operated gadgets, though their fuses may be tiny and
Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting                 53

soldered to the board. A blown fuse pretty much always means a short somewhere
inside, so don’t expect much merely by changing the fuse. Most likely, it’ll blow again
immediately. Still, give it a try, just in case. Be sure to use the same amperage rating
for your new fuse; using a bigger one is asking for trouble in the form of excessive
current draw and more cooked parts, and a smaller one may blow even if the circuit is
working fine. And no matter how tempting it might be, do not bypass the fuse, or you
will almost certainly do much more damage to the circuitry than already exists. Those
fuses are there for a reason, and that reason is protection.
     Though nontechnical types tend to think that truly dead machines are the most
badly damaged and least worth fixing, the opposite is usually true. Total loss of
activity typically indicates a power supply failure or a shorted part that has blown the
fuse. In other words, easy pickings. The really tricky cases are the ones where the
thing almost works right, but not quite, or it works fine sometimes and malfunctions
only if you turn it facing south during a full moon on a Tuesday. Those are the unruly
beasts that may cause you to emit words you don’t want your kids to hear.

If the product has a display, is there anything on it? Although a dead display can
be caused by many things, the condition usually indicates that the microprocessor at
the heart of the digital control system isn’t running. Micros rarely fail, except in cases
of electrical abuse like lightning strikes or severe static electricity. The most frequent
reasons for a stopped microprocessor are lack of proper power supply or a clock
crystal that isn’t oscillating.
     If the display is there but isn’t normal, that’s a sign that some other issue in
the digital system is scrambling the data going to it. If it’s a simple system in which
the microprocessor directly drives the display, the micro still might be stopped or
damaged. If there’s a display driver chip between the micro and the LCD, it may be
bad. When the unit responds to commands but has a scrambled display, the micro is
probably okay. If everything is locked up, suspect the micro or its support circuitry.

Does it work when cold and then quit after it warms up? Thermal behavior can
be caused by bad solder joints, flaky semiconductors and bad capacitors. It usually
manifests as failure after warm-up, but now and then it’s the other way around, with
proper operation commencing only after the unit has been on for awhile. Again, the
problem is not a blown part.

Does tapping on it affect its operation? If so, there’s a poor connection somewhere.
Typically it’s a cold solder joint or an oxidized connector. Cracked circuit board traces
used to be fairly common, but they’re quite unusual now, except in cases of physical
abuse. Faulty conductive-glue layer interconnects can make boards tap-sensitive. On
very rare occasions, the bad connection may be inside a transistor, and I once found
one inside an intermediate frequency (IF) transformer in a radio receiver.

Eliminating variables If the device runs off an AC adapter, try substituting your
bench power supply, being careful of polarity, as discussed in Chapter 3. If the unit
can operate from batteries, put some in and see what happens. The remote control
54   How to Diagnose and Fix Everything Electronic

     won’t turn it on? Try using the front panel buttons. Even if these attempts don’t
     restore operation, at least you’ll know what isn’t causing the trouble.
         Speaking of remotes, they can go wild and emit continuous commands, driving
     the micro in the product out of its little silicon mind and locking out all other attempts
     at operation. The situation usually occurs when liquid has been spilled on the remote,
     causing one or more of the keys to short out. The remote thinks a key is being pressed
     and sends data ad infinitum. To be sure that isn’t the problem, remove the batteries
     from the remote and see if the symptoms disappear.


     Use Your Noodle
     Once you’ve tried these preliminary experiments, think logically about their results
     and you will probably have a pretty good sense of where to poke your scope probe
     first. Let’s look at some real-life cases from beginning to end, and how this approach
     helped get me started in the right direction.

     Stereoless Receiver
     The unit was a fairly high-end stereo receiver with a dead left channel that nobody
     in the shop could bring back to life. Eventually they’d given up, and the set had
     languished on the shelf for two years by the time it and I met. The shop’s owner
     handed it to me as an employment test. If I could fix that one, I was in. The smug look
     on his face told me I was in for a challenge.
          I saw no evidence of obvious damage or abuse, so I hooked up a pair of speakers,
     connected a CD player for a signal source, and fired it up. My initial evaluation was
     that the power supply had to be okay because the right channel worked fine. The
     front panel lit up and the unit seemed to operate normally, other than its having a
     stubborn case of mono. I hooked a clip lead to the antenna terminal and tried FM
     reception, thinking that the trouble might be in the input switching circuitry feeding
     audio from the input jacks to the amplifier stages. Nope, FM sounded great, but still
     from only one channel.
          There was no hum in either channel’s output, so the power supply wasn’t being
     bogged down by a short someplace. (A loudly humming channel with no audio is
     classically indicative of a shorted output transistor.) I plugged in headphones, because
     sometimes amplifiers with bad output stages can drive a little bit of distorted signal into
     headphones, though they can’t power a speaker. I kept the cans off my ears, as always,
     just in case the thing blasted me with punishing volume. There was no difference
     this time; I couldn’t hear a hint of audio from the bad channel, even with the balance
     control turned all the way to that side. It was as quiet as a mouse. A dead mouse.
          I’d eliminated as many variables as I could, so it was time to open ’er up. Several
     techs had taken their best shots at the poor thing, and evidence of their endeavors was
     all over the inside. The output transistors had been changed and large components in
     the power supply resoldered. Other solder work indicated that resistors in and near
     the bad channel’s output stage had been pulled and tested. The focus clearly had been
     toward the output stage, which very often dies in audio amps and is where most techs
     look first. It made sense, but it hadn’t done any good this time.
Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting                 55

     Thanks to the working channel, I didn’t head straight to the power supply. Since
the other guys had replaced the output transistors, I didn’t bother to check those
either. Instead, I stuck my scope probe on the signal line feeding the output stage, and
there was no audio signal. Thus, the trouble was farther back in the chain toward the
input stages someplace, and everybody had been hunting in the wrong place!
     I looked closely at a few small-signal transistors and traced their connections
between stages. Some amplifiers are capacitively coupled (there’s a capacitor between
each stage), while others are directly or resistively coupled. The direct and resistive
styles are also called DC coupling, because the voltages on one stage get passed to the
next. It’s a tougher type of circuit to design, but it results in superior sound. So most
good audio gear works that way, and I expected to see that kind of circuitry here.
     I wasn’t disappointed; this baby had resistors between stages, but no capacitors.
Thus, the DC voltage levels on one stage could affect those on the succeeding stages.
A little light was beginning to glow in the back of my mind, but I needed to take a few
measurements before coming to any conclusions.
     I went all the way back to the first stage, finding it by tracing the line from the
input jack, through the selector switches and to the amplifier board. I had a known
good channel to use as a reference, so I fed the same audio signal to both sides, using
a “Y” adapter cable. Setting the scope for dual trace display and the same voltage
range on both input channels, I compared the outputs from the receiver’s first left
and right channel stages. They looked identical. Same signal levels, same DC voltage.
I went to the next stage. The good channel showed 1-volt DC at that stage’s output,
while the bad side only had about 0.5 volts, with the same audio signal riding on both.
Hmmm…could such a small difference matter? Half a crummy volt? In a DC-coupled
amplifier, you bet it could! Transistors need a “bias,” which is a little bit of DC to keep
them turned on, at their bases (input terminals), and not having a high enough bias
will make them cut off, unable to pass any signal. I checked the next stage in the bad
channel, and its output was dead, just a sad, flat line on my scope. Without proper
bias, the stage was completely cut off. There was the trouble! But why?
     I went back to the stage with lower DC output and checked the voltages and
signals on the transistor’s other terminals. They matched those of the good channel.
Only the output was different. So, most likely, the transistor was just dropping too
much voltage. In other words, a bad transistor. A whole 25 cents’ worth of mysterious
mischief that had stymied an entire shop, simply because it wasn’t the usual problem.
I popped in a new transistor, and voilà! The entire channel came to life and worked
perfectly. Just to be sure all was well, I checked the previously dead stage’s output
levels, and both the signal and DC level matched those of the good channel. Case
closed. I got a few open-mouthed stares from the other techs over that repair, along
with an offer of full-time employment at the shop. I decided not to work there, but the
episode left me feeling like Sherlock Holmes solving a perplexing crime. All I needed
was a pipe and an English accent. “Elementary, my dear Watt-son!”

Silent Shortwave
A friend brought me this set after buying it for very little, knowing it didn’t work but
badly wanting it to, as he’d always longed for one of these models and they were hard
56   How to Diagnose and Fix Everything Electronic

     to find. One of the better digitally tuned shortwave receivers, this portable radio had
     no reception at all. It wasn’t dead, though; the display came up normally, and a little
     hiss came from the speaker. Logic brain, spring into action! Where should I start?
          The first thing I did was try the various bands. AM, nada. Shortwave, same. FM…
     hey, the FM worked! Sounded great. The FM band is at a much higher frequency
     and uses a different kind of signal than AM and shortwave (which is also AM), so
     all multiband radios have separate stages dedicated to FM reception, and clearly
     they were fine. The audio and some other stages are shared, though, so the working
     FM also confirmed that the power regulation, digital control and audio stages
     were all functioning. Thus, the trouble had to be in the RF (radio frequency) or IF
     (intermediate frequency) stages of the shortwave section, which also handled AM, or
     in the digital frequency synthesizer that controlled the tuning.
          I discounted the frequency synthesizer because the FM worked. There still could
     have been trouble there, but it wasn’t suspect number one. Let’s see, the synthesizer
     generates the oscillator signal that mixes with the incoming radio signal from the
     antenna, resulting in the IF signal, which is then amplified by the IF stages. Then, in
     good radios like this set, that signal is mixed with yet another oscillator, this one of
     fixed frequency, to create a lower-frequency IF signal that passes through yet more
     amplifier stages before it is demodulated into audio.
          The trouble could have been anywhere along that chain, but experience reminded
     me to check that the fixed oscillator, called the second local oscillator, was actually
     running. Back in the ’70s, when I’d worked in the service department of a large
     consumer electronics chain, tons of CB radios had come in with dead receivers,
     thanks to a bad batch of oscillator crystals. We’d change ’em and be done in a jiffy,
     fixing the units without even having to troubleshoot them, since they always had the
     same problem. I did so many of them that the issue of a dead second local became
     permanently embedded in my mind. I looked for this set’s crystal and touched each
     end with my scope probe, checking for a nice sine wave of a few volts. Nothing. The
     oscillator was not running. Aha!
          Sometimes weak crystals can be jolted into operating by adding some capacitance
     to one end, increasing the voltage drop across the crystal because of the extra load and
     making it vibrate a little harder. So, I touched my finger to each lead of the crystal,
     one lead at a time, with another finger touching circuit ground via a metal shield,
     employing my hand as a capacitor. This was all very low-voltage, battery-operated
     stuff, and it was safe to do that. The first try, nothing. The second, wham! The radio
     sprang to life and the BBC boomed in loud and clear from thousands of miles away.
     I let go and silence filled the room again. Ah, a bad crystal, and this one would have to
     be ordered from Japan. I tried resoldering it, just in case it had a cold joint. No luck.
     Then, glancing at my friend’s glum expression of disappointment that a new crystal
     would have to be procured from halfway around the world—restoration of the radio
     would be months away—I decided to grab a magnifying glass and take a close look at
     the surrounding components. I spied a tiny surface-mount capacitor connected from
     one end of the crystal to ground, performing essentially the same function my finger
     had. The solder joint on that one looked awfully dull. I resoldered it and the radio
     starting playing its little heart out. “This is London calling. And now the news….”
     Cost: zero. Grin on elated friend’s face: priceless.
Chapter 4     I Fix, Therefore I Am: The Philosophy of Troubleshooting                 57

The Pooped Projector
How about a nice, high-resolution DLP video projector, with plenty of lamp life left,
for $20? Sure, we’d all go for that, right? Oh, there’s one small catch: it doesn’t work!
     I snapped up this craigslist puppy because I knew from the history of its failure
exactly what was wrong before I ever saw it. The owner told me that it had started
turning itself off randomly and becoming difficult to turn on. Eventually it stopped
responding altogether. Now what could possibly cause that? Obviously, it couldn’t be
a blown part. You guessed it: a classic electrolytic capacitor failure. I could picture
just what it would look like with its bulging top. I figured it’d be at the output of the
internal switching power supply, probably near the DC output end of the board.
     Got it home, opened it up, and there was the cap, precisely as I’d pictured it,
bulge and all. It was even where I’d expected it. I changed the part with an exact
replacement I found on one of my scrap boards, a power supply from a computer.
Fired up the projector and she was good to go, with a sharp, bright picture.
     While I gave it the bench test, I checked online and found numerous complaints
of the same problem in this model, along with various lay diagnoses, including some
wacky guesses and the correct answer. The design kept the power supply turned on
at all times, stressing that particular cap and causing it to fail after a couple of years,
regardless of how much use the projector got. I keep my unit unplugged when I’m not
running it, so it should last for a long, long time.
     How can you beat a $20 video projector? And that, gentle reader, is why repairing
electronics is not just fun, it’s incredibly economical.

Chatterbox DVD Player
This portable DVD player came from the carcass pile at a repair facility for which I
worked part time. The machine, one of the better brands, had been a warranty claim,
and nobody could fix the thing, so it had been replaced and kept for parts. With its
5-inch widescreen LCD, the player looked kinda cute, and it seemed a shame to cut it
up. The shop’s owner didn’t care if I took it home, so I did. I had no idea what might
be wrong with it, but the price was right.
     It appeared intact, so I hooked up my bench supply and flipped on the juice. The
screen lit up and the mechanism immediately started making a noise like a machine
gun! I killed the power in a hurry, because I knew what that rapid-fire sound was.
     Disc players use leaf switches to sense when the laser head has returned fully to
the initial position at the inside of the disc, where it needs to go to begin the start-up
sequence leading to disc playback. The “rat-a-tat” noise was a clear indication that the
micro didn’t know the mechanical limit had been reached. The unit was cranking its
sled motor indefinitely, grinding the nylon gears against each other until they slipped,
over and over. I could just imagine the toothless mess it might make of those delicate
plastic parts if I let it run for very long. Yikes.
     On opening the player up, I looked for the typical leaf switch assembly and
couldn’t find one! Did this model use optical sensing? There was no trace of that
either. I gently turned the sled motor’s gear and moved the head away from the
starting position, but I still couldn’t see a switch. Finally, I removed the entire spindle
58   How to Diagnose and Fix Everything Electronic

     assembly, and there it was, a tiny leaf switch hidden underneath the disc motor. It
     looked fine, though. Why wasn’t it being tripped? Or maybe it was, and its signal
     wasn’t getting back to the micro for some reason. Or perhaps the micro was bad….
          I forced myself off the trail of wild imagination and back onto a path of pursuit.
     The simplest explanation was that the switch must not be getting pressed far enough
     to work. I disconnected one wire from the leaf switch and connected my DMM across
     it, watching for the resistance to change from infinite (an open circuit) to near-zero
     (a closed one) as I slowly turned the gear to move the head back toward the switch.
     The head hit its mechanical limit and would go no farther, but the switch never
     closed. That was the problem, all right.
          After moving the head away again, I could see why, and it was so silly that I couldn’t
     imagine why nobody had caught it. The little metal arm on the laser head that pressed
     on the switch was bent—not a lot, but just enough to keep it from pushing the leaf
     far enough to contact its mate. I bent the arm back ever so slightly, and I had a DVD
     player! Almost. Alas, the disc spindle assembly’s three mounting screws also worked
     to set the disc alignment perpendicular to the optical head as it traversed the radius of
     the disc, and I’d had to unscrew them to remove the spindle. Any significant tilt would
     cause the reflected laser beam to miss the center of the head’s lens, resulting in poor
     tracking and skipping. And, with the alignment scrambled, it did. I found the proper
     test point to use for observing the head’s output signal (we’ll explore how to do that in
     Chapter 14), scoped it and redid the alignment, carefully adjusting those three screws
     until I got a good signal no matter what part of the disc I played. Making me mess up a
     critical alignment to reach the leaf switch—talk about poor design!
          I won’t mention the manufacturer’s name, but I’d seen flimsy metal parts in
     some of their other products, so it wasn’t terribly surprising to find one here too. This
     particular player went on to develop a baffling, chronic problem with the ribbon cable
     going to its disc motor, causing it to fail to spin the disc fast enough, resulting in an
     error message and no playback. I kept cleaning the ribbon’s contacts and reseating the
     connector at the circuit board end, and it would work for a few months before failing
     again. Finally, I checked the other end of the cable, which had looked okay, and that
     was the real trouble; I’d just been wiggling it a little while working on the wrong end,
     and the movement had helped its connection for a short time. I cleaned and reseated
     that end, and the unit works to this day. Another mystery solved, another lesson
     learned in never assuming anything, and another fun freebie.
Chapter                    5
   Naming Names: Important
   Terms, Concepts and
   Building Blocks

   W      hile many different terms are used to describe electrons and their behavior, you
          will encounter a core set, common to all electronics, in your repair work. Some
   deal with electrical units, some with parts and their characteristics, some with circuit
   concepts, and others with hip tech slang. (Okay, you can stop laughing now!) Others
   describe frequently employed circuits used as the building blocks of many products.
   Getting familiar with these terms is crucial to your understanding the rest of this book,
   so let’s look them over before moving on to Chapter 6.
        We’ll touch briefly on the most vital terms here; for more, and greater detail, check
   out the Glossary at the back of the book. You’d be doing yourself a favor to read the
   entire Glossary, rather than just using it for reference. Otherwise, you’ll find yourself
   flipping pages back and forth a great deal as you read on. And believe me, you don’t
   want to miss the definition of magic smoke.



Electrical Concepts
   Being an intangible essence, electrical energy must be described indirectly by
   its properties. It possesses quite a number of them, and many famous scientists
   have teased them out with clever observations throughout the last few centuries.
   Experiments with electricity have gone on since the 1700s, when Ben Franklin
   played with lightning and miraculously lived to write about it—talk about conducting
   an experiment! Volta and Ampère built batteries and watched how their mysterious
   output affected wires, compasses and frogs’ legs. Ohm quantified electrical resistance,
   and many others contributed crucial insights into this amazing natural phenomenon’s
   seemingly bizarre behavior.


                                                                                          59
60   How to Diagnose and Fix Everything Electronic

          Perhaps the most valuable discovery was that electricity is a two-quantity form
     of energy. Its total power, or ability to do something like light a lamp, spin a motor
     or push a speaker cone, has two parts: how much of it there is, and how strongly
     it pushes.
          The ampere, or amp for short, is a measure of how much electrical current is moving
     through a circuit. Interestingly, actual electrons don’t travel very fast at all and are not
     what moves through the wires, semiconductors and other parts of a device. Rather, their
     charge state gets transferred from atom to atom, raising the energy level of each one’s
     own electrons, thus passing the current along. Imagine throwing a stone into a pond and
     watching the resulting wave. The wave propagates outward, but do the atoms of water
     at the center, where you threw the rock, actually wind up at the edge of the pond? No;
     they hardly move at all. They just push against the atoms next to them, transferring the
     stone’s mechanical energy from one to the next.
          The amount of current, or number of amperes moving through a circuit, has
     nothing to do with how hard they push, just as the amount of water in a hose has
     no relation to how much pressure is behind it. The pressure is what we call volts,
     a measure of how high each electron’s energy state rises. Volts tell you how much
     pressure is pushing the amps through the circuit. In fact, voltage is sometimes referred
     to as electrical pressure, or electromotive force. It is unrelated to how much electricity
     there is, just as the pressure in a hose doesn’t tell you how much water is present. Volts
     propel current through the circuit. After all, without pressure, the water will just sit in
     the hose, going nowhere, right?
          The hose isn’t infinitely large, of course, and doesn’t permit a perfectly free flow.
     Friction opposes and limits the motion. When the “hose” is a wire, that means it has
     resistance. Resistance is basically friction at the atomic level, and the energy lost to it
     from electrons and atoms rubbing against each other is converted to heat. The term for
     resistance is ohms, after the man who deduced the relationship between current, voltage
     and resistance. We call his crucial insight into electrical behavior Ohm’s Law. If you hate
     math and don’t want to memorize formulas, at least get the hang of this rather simple
     one; it’s the most important, useful relation in all of the electrical arts, and grasping its
     essence will greatly aid your troubleshooting. See Ohm’s Law in the Glossary.
          When you put voltage and current together, you get the total picture of the power of
     the power, so to speak. We call that watts, and it describes how much work the energy
     can do. Determining watts is simple: just multiply the volts times the amps. So, 25 volts
     at 4 amps equals 100 watts, and so does 5 volts at 20 amps. Either arrangement could be
     converted to the same amount of mechanical work or produce the same light or heat.
          As it comes from a battery, electricity is in the form of direct current, meaning it
     moves only in one direction. The side of the battery with excess electron charge is
     called negative, while the side with a lack of it is called positive. Thus, by definition,
     current passes from negative to positive as it attempts to correct the imbalance of
     charges. Why not the other way around? I suppose we could have named either
     terminal whatever we wanted, but those names were known in Franklin’s time and
     have persisted. And they relate to our modern model of the atom, with the electron’s
     negative charges, so I doubt anyone’s going to change them.
Chapter 5    Naming Names: Important Terms, Concepts and Building Blocks                           61

           Flipping the polarity, or direction of current, back and forth turns out to create
      many useful effects, from easing long-distance power transmission to the magic of
      radio signal propagation. That’s alternating current, or AC, and you’ll see it in just
      about everything. How fast you flip it is the frequency, specified in hertz (Hz). The old
      term was cycles per second. It’s nice to have one word for it, don’t you think? I wonder
      why we don’t have one for speed, instead of miles per hour. We should call them
      glorphs. “I’m sorry, sir, you were going 45 glorphs in the 25 glorph zone. License and
      registration, please.”
           When you put two conductive plates in proximity to each other and apply voltage,
      they talk to one another in a peculiar way. A charge builds up on either side of the
      insulator between them, and that charge can be taken out and turned back into current.
      We call that phenomenon capacitance, and the parts doing the job are capacitors.
      Essentially, capacitors act like little storage wells of electricity.
           Electricity and magnetism are very related things, and they interact with each other.
      In fact, one can be turned into the other quite easily, by passing a current through a
      coil of wire or by moving a magnet in a coil of wire. Passing current through the coil
      generates a magnetic field, and moving a magnet through a coil generates current.
      When a coil generates a magnetic field and then the direction of applied current
      reverses, the field collapses on the coil and generates current in it in the opposite
      direction to the current that created the field. Essentially, the coil stores some energy
      in the magnetic field and then puts it back into the circuit, but going the other way.
      That behavior is called inductance, and it has all kinds of very important implications in
      alternating-current circuits. A coil used that way is an inductor. Two different-sized coils
      wound on a common metal core can be used to transform one combination of current
      and voltage into another, with the magnetic field created by one generating current in
      the other. That’s a transformer.
           The effect capacitors and inductors have on AC currents is called reactance, and
      the combination of capacitive reactance, inductive reactance and resistance is known
      as impedance. That’s an especially apt term because it quantifies the amount the circuit
      impedes the passage of the AC current going through it. Though it doesn’t behave
      exactly like resistance—it’s frequency-dependent, for instance—impedance is similar to
      resistance for AC current and is specified in ohms, just like pure resistance.



Circuit Concepts
      When you wire up a bunch of parts such that current can pass through them and return
      to its point of origin, you create a circuit. The circuit concept is central to all electronics,
      and virtually every device that does anything is part of one. So, naturally, lots of terms
      are used to describe the functions and characteristics of circuits and the signals that flow
      through them.
           When two or more circuit elements (components) are wired so that the current has
      to pass through one of them to reach the other, they’re in series. Examples of things in
      series are fuses and switches; nothing can reach the rest of the circuit without passing
      through them first. It makes sense that the current through each element would have
62   How to Diagnose and Fix Everything Electronic

     to be the same, since the amount of electricity reaching the return end of the power
     source has to equal what left it in the first place. Indeed, that’s true. The current that
     passes through each element of a series circuit is always equal.
           Still, energy has to be used in order for the part to do anything, so something has
     to give. What changes is the voltage. Each element drops the voltage, essentially using
     up some of the electrical pressure, until the total drop equals the applied voltage. The
     elements don’t necessarily all drop the same amount of voltage, though. As Ohm so
     cleverly figured out, the amount dropped is proportional to the element’s resistance. If
     one element has 20 percent of the total resistance of the circuit, it drops 20 percent of the
     voltage. Another element that has 10 percent of the total resistance drops 10 percent of
     the voltage, and so on. They’ll always add up to 100 percent, right? Thus, all the voltage
     will be dropped by the time the other side of the power source is reached.
           When circuit elements are wired so that multiple components are connected across
     the power source’s two terminals, they are connected in parallel. In this case, each
     one gets the full voltage because nothing is in the way to drop some of it. The amount
     of current passing through each part is proportional to its resistance, regardless of
     the other parts also connected. Basically, they have no reason to notice each other. If
     you measure the total current passing through a parallel circuit, it’ll add up to all the
     currents going through each leg, or element. A parallel circuit’s conditions are exactly
     opposite to those of a series circuit: the voltage is constant but the current varies.
           Circuits with a path from one end of the power source to the other are said to be
     complete, or closed. That’s the normal operating state; unless a circuit is closed, nothing
     flows and nothing happens. When there’s a break in the path, perhaps from a switch in
     the “off” position or a blown fuse, energy flow stops and the circuit is open. Any failed
     component no longer capable of passing current is considered open as well.
           A condition causing part or all of a circuit to be bypassed, so that current passes
     straight to the other end of the power source, is called a short circuit, and the parts
     causing the detour are said to be shorted. Certain types of components, especially
     semiconductors, often short when they fail.
           Although the generation and transport system bringing power into your home
     provides AC, electronics really can’t use the stuff. Just as you couldn’t drink from a cup
     swinging back and forth, circuits can’t take AC power and amplify or process signals
     with it; the changes in the power itself would show up in the output. What’s needed is
     a nice, steady cup from which to sip. In other words, smooth DC.
           Once AC is rectified, or converted into one polarity, it’s still a series of waves of
     power going up and down. To smooth it into a steady voltage, some kind of reservoir
     needs to store some of it, so that as the wave strength approaches zero between
     waves, the stored energy can fill in and raise the voltage back up. That reservoir is
     a filter capacitor. It’s just a big capacitor that can store enough energy to do the job,
     momentarily emptying itself to power the rest of the circuit until the next power wave
     fills it back up again.
           As circuits turn on and off and their signals rise and fall, their varying use of current
     can pull the voltage level feeding them up and down, causing corruption of the desired
     signal. Smaller filters called bypass capacitors, placed close to the part of the circuit pulling
     current, store some energy to fill in the gaps, just like the big guns do. The only difference
Chapter 5    Naming Names: Important Terms, Concepts and Building Blocks                         63

      between a filter capacitor and a bypass capacitor is its size. Generally, big ones in power
      supplies are called filter caps, and little ones in signal processing stages are called bypass
      caps. Bypass capacitors are also used to provide a low-impedance AC path to ground
      in parts of amplifiers where that’s needed, without shorting out the DC on the same
      connection.
            The circuitry in an electronic product is not just a huge mishmash of components.
      It is organized into sections and, within those, stages. Each stage performs one function
      of whatever process is required for the device to do its job. A stage might be an audio
      preamplifier (a low-level amplifier), a tone control, a video display driver, a demodulator
      (something that extracts information from a signal), a position detector for a motor, and
      so on. At the heart of each stage are one or more active elements. These are the parts that
      actually do the work and are generally defined as being capable of providing gain, which
      you’ll read about in just a few paragraphs. Supporting the active elements are passive
      components like resistors, capacitors and inductors. Those can alter a signal, but they
      can’t amplify it. Without them, though, the active elements can’t do their jobs.
            Stages feed signals to other stages, until the device finally produces whatever
      output is desired. The components passing the signal from one stage to the next are
      called coupling elements, and are usually capacitors, resistors or transformers.



Signal Concepts
      Signals are voltages varying in strength, or amplitude, to convey some kind of information.
      Analog signals vary the voltage in a pattern resembling the information itself. For
      instance, the output of an audio amplifier looks like a graph of the original pressure
      waves of sound in the air that struck the microphone. A video circuit’s signal has
      varying voltages representing the brightness of each dot on the screen, with a rather
      complex method of conveying color information and synchronizing the spots to the
      correct place in the picture. Its graph doesn’t look like an image, but it’s still an analog
      signal, with fine voltage gradations portraying the changing picture information.
           The graph of a signal is called its waveform. Every time the waveform repeats, that’s
      one cycle. The number of cycles occurring in 1 second is the waveform’s frequency, and
      the amount of time each cycle takes is its period. Because the voltage varies over time, it
      is a mathematical function, meaning that its lines can’t cross over themselves. Graphed
      from left to right, as they are on an oscilloscope, the level at each successive moment is
      to the right of the preceding moment’s portrayal.
           The purest, most basic waveform is the sine wave (Figure 5-1). It is the building
      block from which all other waves can be created, and it has no harmonics, or energy
      at frequencies that are multiples of the wave’s frequency. A sine wave sounds like a
      pure tone, with no characteristics suggesting any particular musical instrument or tone
      color. In fact, no non-electronic musical instrument produces sine waves, though some
      registers on the flute come close. A tuning fork comes closer.
           When a signal rapidly switches between all the way on and all the way off, it
      assumes a square shape and is called, appropriately, a square wave (Figure 5-2).
      Close enough examination will reveal that the on/off transitions aren’t entirely
64   How to Diagnose and Fix Everything Electronic




                                 Figure 5-1     Sine wave



     vertical, because it takes time for the state to change. Thus no square wave is truly
     square. The time it takes the transition to rise from 10 percent to 90 percent of its final
     state is the rise time. Going back down, it’s the fall time.
          The percentage of time spent in the “on” state, compared to the “off” state, is called
     the duty cycle and can be altered to represent information or control a motor or a voltage
     regulator, using a technique called pulse-width modulation, or PWM. Unlike sine waves,
     square waves contain harmonic energy. They include odd harmonics, but not even ones.
     That is, there is energy at three, five and seven times the frequency, but not at two, four
     and six times.
          A signal used in applications requiring something to move and then quickly snap
     back is the sawtooth wave, so named for its obvious resemblance to its namesake
     (Figure 5-3). Oscilloscopes and CRT TVs use sawtooth waves to sweep the beam
     across the screen and then have it rapidly return. Other circuits, including servos
     (motor position controllers) in video tape recorders, use sawtooth waves too.
     Sawtooth waves include both odd and even harmonic energy.




                                 Figure 5-2     Square wave
Chapter 5    Naming Names: Important Terms, Concepts and Building Blocks                          65




                                   Figure 5-3      Sawtooth wave


           The relative position in time of two waveforms is called their phase relationship and
      is expressed in degrees. As with a circle, 360 degrees represent one cycle of a waveform,
      regardless of how long that cycle takes. Two waveforms offset by half a cycle are
      180 degrees out of phase. When there is no offset, the waveforms are in phase.
           Digital signals are entirely different. Using pulses resembling square waves, digital
      information is always in one of two states, on or off, representing the binary numbers
      1 and 0. That has tremendous advantages over the analog method, because keeping
      track of those two states is a lot easier than accurately moving and processing a voltage
      with infinitely fine gradations. Noise in a digital channel has no effect at all until it’s
      so bad that the two states can’t be found, while noise in an analog channel is very hard
      to separate from the desired signal and corrupts it badly. That’s why scratches on an
      analog LP record create clicks and pops in the audio, while scratches on a CD don’t. All
      circuits introduce some noise, so the digital method is less susceptible to degradation
      as it moves through various processes. Digital data is also much easier to store and
      manipulate, again because it has only two states to worry about.
           The world of sound and light is inherently analog, though; nothing in nature exists
      only as ons and offs! To digitize natural phenomena like sounds and images, an ADC, or
      analog-to-digital converter, is used to chop the analog information into a rapid series of
      samples, or measurements, which are then encoded, one by one, into the binary 1’s and
      0’s of digital data. Conversion is a complicated process that introduces quality limitations
      of its own, so digital is no more perfect than is analog. Digital’s imperfections are different,
      though, and generally less objectionable.



Building Blocks
      There’s one heck of a variety of circuits out there! For any given function, a designer
      can find lots of ways to build something that works. While the circuitry “wheels”
      get reinvented all the time, they’re all round and they all spin, so common circuit
      configurations are found in pretty much all products. Sometimes they have significant
66   How to Diagnose and Fix Everything Electronic

     variations, but they’re still basically the same old thing and can be recognized easily once
     you get familiar with them. Let’s look at some common circuits you’re likely to find.
          The basic circuit at the heart of most stages is an amplifier of some kind.
     Amplifiers have gain, which means they take steady DC from the power supply and
     shape it into a replica of an incoming signal, only bigger. That “bigger” can be in terms
     of voltage, current or both. Most voltage amplifiers also flip the signal upside down, or
     invert it. Sometimes that’s important to the circuit’s operation, but much of the time it’s
     just an irrelevant consequence of how voltage amplifier stages are configured.
          A very common type of current amplifier is the emitter follower. This one takes its
     output from the emitter of a transistor, and the amplified signal mimics, or follows, the
     input signal, without inversion or change in voltage swing. Only the amount of current
     the signal can pump into a load is increased.
          When amplifying analog signals, linearity, the ability to mimic the changes in
     the input signal faithfully without distortion, is a critical design parameter. The term
     comes from the graph that results if you plot the input signal against the amplified
     output. In a truly linear circuit, you get a straight line. The more gain, the more the
     line points upward, but it’s still straight, indicating a ratio of input to output that
     doesn’t change as the signal’s voltage wiggles up and down. If the amplifier is driven
     past the point that its output reaches the power supply voltages, the transistors will be
     all the way on or all the way off during signal extremes, resulting in an output that no
     longer accurately follows the input. That’s serious nonlinearity, also known as clipping
     because it clips off the tops and bottoms of the waveform; the amplifier simply can’t
     go any farther. If you’ve ever turned up a stereo loud enough to hear ugly distortion,
     you’ve experienced clipping. Even a small amplifier driven to clipping can burn out
     the tweeters in a pair of speakers that normally could take the full power of a bigger
     amplifier without harm. The high-frequency content of a clipped waveform is much
     greater than that of a linearly amplified signal, thanks to the steep edges of the clipped
     area, so it drives disproportionate power into the tweeters. I’ve seen it happen. It can
     injure your ears, too.
          Most high-fidelity audio amplifiers, and even many small ones of the sort used
     in pocket radios and MP3 players, use a complementary design, referred to as Class
     AB. Complementary amplifiers split the audio waveform into its negative-going and
     positive-going halves and amplify each half separately. Then they combine the two
     halves at the output, rebuilding the waveform. Why do that? The technique allows for
     excellent efficiency because almost no power is dissipated when the waveform is near
     the zero voltage level. Only when the input signal gets big does the amplifier draw a lot
     of power, keeping power usage proportional to output.
          In a non-complementary design, the amplifier has to be biased to set its output
     halfway between ground and the power supply voltage when no signal is applied, so
     that the negative and positive peaks of signals can make the output swing both higher
     and lower in step with them. It sits there eating half the available power at all times.
     Some high-end amplifiers, called Class A, actually do it that way to avoid certain subtle
     distortions associated with splitting and recombining the waveform. Those amplifiers
     sound great but run very hot and waste a lot of power.
Chapter 5   Naming Names: Important Terms, Concepts and Building Blocks                        67

           Tuned amplifiers use resonant circuits to select a particular frequency or range
      of frequencies to amplify, rejecting others. They’re used in radio and TV receivers
      to separate and boost incoming signals. See resonance in the Glossary. The very first
      amplifiers in a receiver that strengthen the weak signals from the antenna are called the
      front end. Some are tuned, and some aren’t, depending on the design. Later amplifier
      stages, operating at a fixed frequency to which all incoming signals are converted, are
      called intermediate frequency, or IF, amplifiers. Those are always tuned, and they provide
      most of a receiver’s selectivity, or ability to separate stations.
           Even digital gates in integrated circuits are amplifiers. They take in digital pulses
      and amplify them enough to ensure that they swing all the way to the supply voltage
      and ground, making up for any losses that may have occurred and preventing them
      from accumulating until the pulses can no longer be reliably processed. These
      amplifiers are deliberately nonlinear; the only desired states are fully saturated, or
      all the way on, and fully cut off. Their linear region, where small changes in the input
      signal would be faithfully amplified, is made as narrow as possible. Such small wiggles
      in digital signals only represent noise anyway.
           Oscillators generate their own signals. They have many uses, including providing
      timing pulses to, or clocking, microprocessors, generating tones, mixing with radio
      signals to convert their frequencies, and lots more. An oscillator is basically an
      amplifier with its output fed back to its input in phase, reinforcing the input and
      sending the signal around and around again indefinitely.
           Oscillators can be designed to produce any of the basic waveforms. In analog signal
      processing circuits, sine wave oscillators are often the most useful, thanks to their purity.
      Sawtooth wave oscillators are used to drive electron beams across CRTs and any time
      something needs to be swept and then quickly returned to its starting point. With digital
      circuits, which operate with pulses, square waves are the order of the day.
           The significant challenge with most oscillators is setting the frequency and keeping
      it constant. When only a single frequency is needed, a quartz crystal or ceramic
      resonator can keep the oscillator very accurate. These parts resonate mechanically
      at the molecular level, and they’re dimensionally stable, so they drift very little with
      temperature. The tradeoff is that a given crystal can generate only one frequency.
           When frequency variability is required, as in a radio tuner, simple resonant circuits
      like inductor/capacitor combinations work but are not terribly stable, especially
      regarding thermal drift.
           While early radios could tolerate some drift, today’s high-precision systems simply
      can’t. Do you really want to get up to fine-tune your digital HDTV every 20 minutes
      as the receiver drifts off-frequency and the picture drops out? Of course not! The
      receiver has to sit on its tuned frequency the moment you turn it on and stay there
      all day long.
           The digital frequency synthesizer solves the stability problem by providing frequency
      agility while still being referenced to the unvarying frequency of a quartz crystal.
      Pulling that off isn’t simple. There are two basic techniques. In a classic hybrid analog/
      digital synthesizer, an analog oscillator’s frequency is controlled by a voltage from the
      synthesizer. The resulting frequency is digitally divided or multiplied until it matches
      that of the crystal. The two are then compared, and the controlling voltage is adjusted
68   How to Diagnose and Fix Everything Electronic

     to keep them at the same frequency. So, even though the analog oscillator really isn’t
     at the same frequency as the crystal, the comparison circuit thinks it is. This kind of
     circuit is called a phase-locked loop, or PLL.
          Tuning the oscillator is accomplished by changing the ratio of division and
     multiplication, forcing the control voltage to change the true frequency to match the
     result of the division to that of the unvarying crystal. Many receivers have been built
     this way, but the method has a serious downside. In order for the comparison and
     correction process to work, there has to be a little error to correct. The oscillator’s
     frequency wobbles ever so slightly as it drifts and gets corrected, resulting in a
     phenomenon called phase noise. The noise can be kept low, and some very nice
     radios worked just fine with this style of synthesizer, but it caused enough signal
     distortion that better methods were sought.
          As digital technology advanced, chips got fast enough that the oscillator could be
     done away with altogether, and the required output signal could be built directly from
     digital data and converted to analog, in much the same way a CD player rebuilds the
     audio waveform of a music disc from samples. In many applications, this direct digital
     synthesis, or DDS, technique has replaced hybrid analog/digital synthesizer designs. Not
     everywhere, though: you’ll still find PLL synthesizers in lots of UHF and VHF receivers
     because today’s chips are only now getting fast enough to create the required signals
     at such high frequencies. The tipoff is if you see a component on the schematic that
     looks like a combination of a capacitor and a diode. That’s a varactor, or voltage-variable
     capacitor, and it’s what tunes the analog oscillator with the digital part of the circuit’s
     control voltage. Where there’s a varactor, there’s a PLL.
          PLLs are used for other purposes too. Digital data is recovered from media such
     as hard drives and optical discs using a PLL to synchronize the data rate to the circuit
     detecting it. Analog VCRs and camcorders use PLLs to recover the wobbly color
     information from the tape and stabilize it to the very high precision required for proper
     display.
          Servos are a lot like PLLs, except that they slave a motor’s rotational speed and
     phase (position at a given moment) to a reference signal. A servo regulates a DVD
     player’s disc rotation, keeping it at whatever speed is required for a given data rate.
     The rapid lens motions required to focus the laser beam on the microscopic pits and
     track them as they whiz by are also controlled by servos. In a VCR, servos adjust
     capstan motor and head drum rotation, locking them to a signal recorded on the tape,
     so the rotating heads will correctly trace over the recorded tracks.
          Voltage regulators keep power supply voltages constant as current demand varies
     with circuit function. Linear regulators use a series pass transistor as a variable resistor,
     automatically changing the resistance to set the voltage, and dissipating the unwanted
     extra power as heat. They’re simple and effective but also inefficient. Linear regulators
     handling serious power get quite hot.
          Switching regulators use pulses to turn a transistor on and off like a switch, and
     then reconstitute the pulses passed through it back into steady DC power with a
     filter capacitor. Changing the pulse width permits more or less power to get through
     the transistor over a given period of time. Because the transistor is almost always
     completely on or completely off, except for the short moments when it switches states,
Chapter 5   Naming Names: Important Terms, Concepts and Building Blocks                    69

      little power is converted to heat, and there’s no excess to waste, since only as much
      energy as needed is allowed through. Switching regulators are more complicated, and
      they have the potential to generate electrical noise, but their cool-running efficiency
      makes them very desirable, especially in battery-operated devices, where conserving
      power is critical.
            Now that you’ve seen some basic terms, core concepts and circuit building blocks,
      go read the Glossary and see lots more. Seriously. Do it now, before going on to the
      next chapter. I know, “Grrr, this guy is such a nag.” You’ll thank me later. Really, you
      will. And, hey, there’s some cool stuff there. You’ll enjoy it, I promise.
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Chapter                    6
   Working Your Weapons:
   Using Test Equipment

   E    ffective application of test gear is key to your sleuthing success. Especially with
        the oscilloscope, the settings you make while probing around a circuit determine
   what elements of the signals you will see and which ones you may miss. Soldering,
   too, can be more or less effective, depending on your technique. Let’s look at each
   piece of basic test gear and how to use it to your best advantage.



Digital Multimeter
   Digital multimeters (DMMs) are great for measuring things that don’t change quickly.
   Battery and power supply voltage, along with resistance and current, are prime candidates
   for being checked with a DMM. The instrument is less effective for observing changing
   voltages and currents, which look like moving numbers and are tough to interpret.


   Overview
   The DMM’s great advantages over other instruments are its precision and accuracy.
   Even a digital scope has fairly limited resolution; you can’t tell the difference between
   6.1 and 6.13 volts with one very easily, if at all, and measuring resistance and current
   is impossible with normal scope setups.
        All that detail in the meter’s display can get you into trouble, though, if you take
   it too seriously. When interpreting a DMM’s readings, keep in mind that real life
   never quite hits the specs. Don’t expect the numbers you see to be perfect matches
   for specified quantities. If you’re reading a power supply voltage that’s supposed to be
   6 volts, a reading of 6.1 probably isn’t indicative of a circuit fault. The same is true of
   resistance; if the reading is very close, the part is most likely fine. And if the rightmost
   digit wanders around a little bit, that’s due to normal noise levels or the digitizing


                                                                                           71
72   How to Diagnose and Fix Everything Electronic

     noise and error inherent in any digital sampling system. Remember, when a part goes
     bad, it’s not subtle! Real faults show readings far from the correct values.
         Most DMMs run on batteries, and that’s a good thing because it eliminates any
     ground path from the circuit you’re testing back to your house’s electrical system.
     The instrument “floats” relative to what’s being tested (there’s no common ground), so
     you can even take measurements across components when neither point is at circuit
     ground. If your DMM has the option for an AC adapter, don’t use it. Always run your
     DMM on battery power. The batteries will last for hundreds of hours anyway.


     DC Voltage
     To check a circuit point’s voltage, first you must find circuit ground. Usually, it’s the
     metal chassis or metal shields, if there are any. Don’t assume that heatsinks, those
     finned metal structures to which are attached larger transistors, voltage regulators and
     power-handling ICs, are connected to ground! Sometimes they are, sometimes they’re
     not. In switching power supplies, the chopper transistor’s heatsink may have several
     hundred volts on it. You sure don’t want to hook your meter there.
          In some devices, especially small ones like digital cameras, you may find no
     shields, and there’s no metal chassis either. So where is ground? In most cases, the
     negative terminal of the battery will be connected to circuit ground, and you can use
     that. Particularly if you can trace it to a large area of copper foil on the board, it’s a
     fairly safe bet. Also look for electrolytic capacitors in the 100 µf-and-up range with
     voltage ratings lower than 50 volts or so. (See Chapter 7.) Those are most likely power
     supply filter caps, even in battery-operated gear, and their negative terminals will be
     connected to ground. If you see two such identical caps close to each other, the device
     may have a split power supply, with both negative and positive voltages. Trace the
     caps’ terminals and see if the negative lead from one is connected to the same point
     as the positive from the other. Where they meet is probably circuit ground.
          If all else fails, you can use the outer rings of RCA jacks on audio and video gear.
     The only way to get an alligator clip to stay put on one of those jacks is to push half
     of it into the jack, with the other half grabbing the ground ring. It’s better to use an
     input jack, rather than an output, so the part of the clip sticking inside can’t short
     out an output, possibly damaging the circuitry. You can’t hurt an input by shorting it
     to ground.
          Turn on your DMM, set its selector switch to measure DC voltage, and connect
     its negative (black) lead with a clip lead to circuit ground, regardless of whether you
     intend to measure positive or negative voltage. A DMM will accept either polarity;
     measuring negative voltage simply adds a minus sign to the left of the displayed value.
          With power applied to the circuit under test, touch the positive lead’s tip to the
     point you want to measure, being careful not to let it slip and touch anything else.
     Many DMMs are autoranging and will read any voltage up to the instrument’s ratings
     without your having to set anything else. Keep the probe in place until the reading
     settles down; it can take 5 or 10 seconds for the meter to step through its ranges and
     find the appropriate one.
            Chapter 6      Working Your Weapons: Using Test Equipment               73

     If your DMM is not autoranging, start at the highest range and switch the range
down until a proper reading is obtained. If you start at the lowest range and the
voltage you’re measuring happens to be high, you could damage the DMM.
     If you see a nice, steady number somewhere in the voltage range you expect, it’s
safe to assume you have a valid measurement. If, however, you see a moving number
at a very low voltage, you’re probably just reading noise on a dead line, and you may
have found a circuit problem. If you see a voltage in the proper range but it won’t
settle down, that indicates noise on the line, riding along with the voltage. Such a
reading can suggest bad filter capacitors, but only when the point you’re measuring
is supposed to have a clean, stable voltage in the first place. Regulated power supply
output points should be steady, but some other circuit points may carry normal
signals that fool the DMM, causing jumping readings. To see what’s going on with
those, you’ll be using your scope. Generally, electrolytic caps with one lead going to
ground shouldn’t have jumpy readings, since their reason for being in the circuit is to
smooth out the voltage.


AC Voltage
You’ll usually use this as a go/no go measurement. Is the voltage there or not? DMMs
are optimized to read sine waves at the 60-hertz AC line frequency, so the reading
doesn’t mean much if you try to measure an audio signal or the high-frequency pulses
in a switching power supply. Measurements are taken across two points, as with DC
voltage, but in many circuits neither point will be at ground.
     DMMs indicate AC voltage as root-mean-square (RMS), which is a little bit more than
the average voltage in a sine wave when taken over an entire cycle. It’s a useful way
of describing how much power an AC wave will put into a resistive load, compared
to DC power, but it is not a measurement of the actual total voltage swing. The RMS
value is much smaller than the peak-to-peak voltage you’ll see with your scope.
American AC line voltage, for example, is 120V RMS and reads about 340 volts peak-to-
peak on a scope. (If you want to keep your scope, don’t try viewing the AC line with it
unless you have an isolation transformer!)
     For a sine wave, RMS is 0.3535 times the peak-to-peak value. For other waveforms,
it can be quite different, as the time they spend at various percentages of their peak
values varies with the shape of the wave. DMMs are calibrated to calculate RMS for
sine waves, so the reading will be way off for anything else, at least with hobbyist-
grade meters.


Resistance
When measuring resistance, turn off the power to the circuit! The battery in your DMM
supplies the small voltage required to measure resistance, and any other applied
power will incur negative consequences ranging from incorrect readings to a damaged
DMM. In addition to removing the product’s batteries or AC adapter (or unplugging
it from the wall, in the case of AC-operated devices), it pays to check for DC voltage
74   How to Diagnose and Fix Everything Electronic

     across the part you want to measure and to discharge any electrolytic caps that could
     be supplying voltage to the area under test.
          Some resistances can be checked with the parts still connected to the circuit,
     but many cannot because the other parts may provide a current path, confusing the
     DMM and resulting in a reading lower than the correct value. For most resistance
     measurements, you will need to unsolder one end of the component. When one side
     of it goes to ground, leave that side connected and connect your DMM’s negative
     lead to the ground point; it’s just more convenient that way. When neither side is
     grounded, it doesn’t matter which lead you disconnect.
          Set the DMM to read resistance (Ω, or ohms). If it’s autoranging, that’s all you
     need do. Let it step through its ranges, and there’s your answer. If it isn’t autoranging,
     start with the lowest range and work your way up until you get a reading, so you
     won’t risk applying the higher voltages required to get a reading on the upper ranges
     to sensitive parts. DMMs with manual ranges have an “out of range” indicator to
     show when the resistance being measured is higher than what that range can accept,
     usually in the form of the leftmost digit’s blinking a “1.” (If you’re on too high a range,
     you’ll see all 0’s or close to it.)
          With a manually ranging DMM, you can get more detail by using the lowest range
     possible without invoking the out-of-range indicator. For instance, if you are reading a
     10-ohm resistor on the 20 KΩ (20,000 ohm) scale, you’ll see 0.001. If you switch to the
     200-ohm scale, you’ll see 0.100 or thereabouts. If the resistor’s measured value is too
     high by, say, 20 percent, which is a significant amount possibly indicating a bad part,
     it might show 0.120, critical data you’d miss by being at too high a range. Autoranging
     meters always use the lowest possible range, for the most detailed reading.
          Resistance has no polarity, so it doesn’t matter which lead you connect to which
     side of a resistor. If you’re checking the resistance of a diode or other semiconductor, it
     does matter, and you must swap the leads to see which polarity has lower resistance.
     The essence of a semiconductor is that it conducts only in one direction, so a good one
     should have near-infinite resistance one way and low resistance the other. Checking
     semiconductors for resistance with a DMM can yield unpredictable results, though,
     because the applied voltage may or may not be enough to turn the semiconductor on
     and allow current to pass, depending upon the meter’s design. There are better tests
     you can perform on those parts, but a reading of zero or near-zero resistance pretty
     definitively indicates that the component is shorted.


     Continuity
     Continuity simply shows whether a low-resistance path exists, and is intended as a
     “yes or no” answer, rather than as a measurement of the actual resistance. It’s exactly
     like taking a resistance measurement on the lowest scale, except that many meters
     have a handy beeper or buzzer that sounds to indicate continuity, so you don’t even
     have to look up. Use this test for switches and relay contacts, or to see if a wire is
     broken inside its insulation or a connector isn’t making proper contact.
          In many instances, you won’t need to pull one side of the component to check
     continuity, as the surrounding paths will have too much resistance to fool the meter
             Chapter 6      Working Your Weapons: Using Test Equipment                 75

and invalidate the conclusion. There are some exceptions, however, involving items
like transformers, whose coil windings may offer very little resistance and appear as
a near-zero-ohm connection across the part you’re trying to test. If you’re not sure,
pull one lead of the component. And, as with resistance measurement, make sure all
power is off when you do a continuity check!


DC Current
Most DMMs can measure current in amps or milliamps. To measure current, the
meter needs to be connected between (in series with) the power source and the
circuit drawing the power, so that the current will pass through the meter on its way
to the circuit. Thus, neither of the DMM’s leads will be connected to ground. Never
connect your DMM across (in parallel with) a power supply’s output when the meter
is set to read current! Nearly all the supply’s current will go through the meter, and
both the instrument and the power supply may be damaged. The meter, at least,
probably will be.
     Even with the meter properly connected, it’s imperative that you not exceed its
current limit or you will damage the instrument. For many small DMMs, the limit is
200 milliamps (ma), or 0.2 amps. Some offer higher ranges, with a separate terminal
into which you can plug the positive test lead, extending the range to 5 or 10 amps.
     In estimating a device’s potential current draw, take a look at what runs it. If it’s
a small battery, as you might find in an MP3 player or a digital camera, current draw
probably isn’t more than an amp or so. For some devices, it’s much less, in the range
of 100–200 ma. If the unit uses an AC adapter, the adapter should have its maximum
current capability printed on it somewhere, and it’s safe to conclude that the product
requires less than that when operating properly. Some gadgets state their maximum
current requirements on the backs of their cases, too. When they do, they indicate
the maximum current needed under the most demanding conditions—for example,
when a disc drive spins up or a tape mechanism loads—and normal operating current
should be less.
     To take a current measurement, you need to break a connection and insert the
meter in line between the two ends of it. Don’t worry about test lead polarity; all you’ll
get is a minus sign next to the reading should you attach it backward. If you want to
know the current consumption of an entire product, connect the meter between the
positive terminal of the battery or power supply and the rest of the unit. If you want to
measure the current for a particular portion of the circuitry, disconnect whatever feeds
power to it and insert the meter there.
     The DMM measures current by placing a low resistance between the meter’s
leads and measuring the voltage across it. The higher the current, the higher that
voltage will rise. With a big current, the resistance can be very low, and there will still
be enough voltage across it to get a reading. With smaller current, the resistance needs
to be higher to obtain a significant, measurable voltage difference. Thus the higher
ranges place less resistance between the power and the circuit. Start with the meter’s
highest range and work your way down. Using too low a range may impede the
76   How to Diagnose and Fix Everything Electronic

     passage of current enough to affect or even prevent operation of the product. It also
     may heat up the DMM’s internal resistor enough to blow it.
         Current is perhaps the least useful measurement and consequently the one most
     infrequently performed. Now and then, it’s great to know if excessive current is being
     drawn, but heat, smoke and blown fuses usually tell that story anyway. The more
     revealing result is when current isn’t being drawn; that tells you some necessary
     path isn’t there, or that the unit isn’t being turned on. Especially because breaking
     connections to insert the meter is inconvenient, however, you won’t find yourself
     wanting to measure current very often.


     Diode Test
     Some DMMs offer semiconductor junction tests, making them handy for checking diodes
     and certain types of transistors. The measurement is powered by the meter’s battery,
     as with resistance measurements, but it’s taken somewhat differently. Instead of seeing
     how many ohms of resistance a part has, you see the voltage across it. And to complete
     the test, you must reverse the leads and check the flow in the other direction.
          Kill the power and disconnect one end of the component for this test. A good
     silicon diode should show around 0.6 volts in one direction and no continuity at all
     in the other. That lack of flow will be shown as the maximum voltage being applied,
     typically around 1.4 volts. (You can check your meter’s open-circuit value by setting it
     to the diode test without connecting the leads to anything.) If you see 0 volts or near
     that, the part is shorted. To verify, switch the leads and you should see zero in the
     other direction too. If you see 1.4 volts (or whatever your meter’s maximum is) in both
     directions, the part is open, a.k.a. blown. If the meter indicates the normal 0.6 volts
     in the conducting direction but also shows even a slight voltage drop the other way
     around, the diode is leaky and should be replaced.
          Some DMMs perform capacitance, inductance, frequency and other measurements,
     but most don’t. If yours does offer these readings, see the sections on those kinds of
     meters, and the principles will apply.



Oscilloscope
     Back in Chapter 2, I emphasized that the scope is your friend. Now it’s time to get
     acquainted with your new best buddy. This is the most important instrument, so
     learning to use it well is absolutely vital to successful repair work. There’s no need to
     be intimidated by all those knobs and buttons; we’ll go through each one and see how
     it helps you get the job done. Various scope makes and models lay out the controls
     differently, and some call them by slightly different names, but they do the same
     things. Once you get used to operating a scope, you can figure out how to use any
     model without difficulty.
          While the functions of analog and digital scopes are basically the same, each type
     offers a few features unique to its species, along with some characteristic limitations.
             Chapter 6       Working Your Weapons: Using Test Equipment                   77

Let’s look at a scope’s functions and operation, using an analog instrument as an
example, with digital-specific differences noted along the way. Then we’ll review
some important items to keep in mind when working with a digital unit.


Overview
You cannot harm your scope by misadjusting its controls, outside of possibly damaging
the CRT with an extremely high brightness setting. But even that doesn’t happen in an
instant. So have no fear as you play with the knobs, and feel free to experiment and
learn as you go. Just keep the brightness to reasonable levels and you’ll be fine.
     The purpose of a scope is to plot a graph of electrical signals, with horizontal
motion, or deflection, representing time and vertical motion representing signal
voltage. Various controls adjust the speed and the voltage sensitivity so you can scale
a wide range of signals to fit on the display. Others help the scope trigger, or begin
drawing its graph, on a specific point in the signal, for a stable image. Still more let
you perform special tricks helpful in viewing complex signals. Now you know why
a scope has so darned many knobs!
     To get signals on the screen, you will first connect the probe’s ground clip to
circuit ground of the device you’re examining. If circuit ground is connected to the
wall plug’s round ground terminal, that’s fine, but never connect the clip to any
unisolated voltages—that is, points connected to the AC line’s hot or neutral wires.
Doing so presents a serious shock hazard, along with the distinct possibility of
destroying your scope. (This issue crops up mostly when you’re working on switching
power supplies, so read up on them in Chapter 14 carefully before you try to connect
your probe to one.)
     Next, you’ll touch the probe’s tip to the circuit point whose signal you want to
see, and set the vertical, horizontal and trigger controls to scale the signal to fit on the
display and keep it steady. Really, that’s all there is to it. The rest is just details.
     The boxes on the face of the screen, called the graticule, are used for visual estimation
of the signal’s voltage and time parameters, and the vertical and horizontal controls
are calibrated in divs, for divisions. One box equals one division, and the boxes are
subdivided into five equal parts, for an easily visible resolution of 0.2 divisions, or
0.1 divisions if you want to count the spaces in between the subdivision lines.
     So, if the vertical input control is set to 0.5 volts/division, and your signal
occupies two divisions from top to bottom, it’s a 1-volt signal. Similarly, if the time/
div control is set to 0.5 µs (microseconds, or millionths of a second) and one cycle
of your signal occupies two divisions from left to right, it has a period of 1 µs and is
repeating at a rate of about 1 Mhz. (1/period = frequency. Something that occurs
every millionth of a second happens a million times a second, right?)
     Notice I said “about 1 Mhz.” Keep in mind that scopes are not intended for
measurements requiring tremendous precision or accuracy. A few percent is the
best they do. Newer designs offer more accurate time calibration, thanks to digital
generation of their internal timing clocks, but the precision is still low compared to
that of a digital frequency counter’s many digits. Plus, the vertical input specs don’t
approach those of the horizontal, even on digital scopes, because scaling the incoming
78   How to Diagnose and Fix Everything Electronic

     voltage so it can be digitized is still an analog process, with all the attendant drift and
     error inherent in those.
         Though the layout of scopes varies by manufacturer, vertical controls are usually
     near each other, with horizontal ones grouped together somewhere else. Typically,
     vertical stuff is on the left and horizontal is on the right. Other functions, like triggering
     and screen controls, may be anywhere, although screen settings are usually under the
     screen, with triggering near the far right edge of the control panel.
         To begin, find the power button, most likely near the bottom of the screen. Turn
     the scope on and adjust the following controls.


     Screen Settings
     This group of controls is pretty much always located beneath the screen on CRT-type
     scopes. Most scopes with CRTs are analog, but some early digitals use them too.

      •	 Brightness or intensity Set it to midrange. If it seems rather bright, turn it
         down a bit. If you see nothing on the screen, don’t turn the brightness way up.
         Other controls may need to be set before you’ll see a line, or trace, on the display.
         If your scope has dual brightness controls for A and B, use A. The B control is for
         delayed sweep operation, which we will explore a little later on.
      •	 Focus If you can already see a trace on the screen, adjust the focus for the
         sharpest line. Otherwise, set it to midrange.
      •	 Astigmatism or astig Set it to midrange. Some scopes don’t have an astig
         control, but most do.


     Vertical Settings
      •	 Vertical mode This control or set of buttons may be anywhere on the scope,
         but it’s usually near the vertical channels’ controls. Set it to channel 1.
      •	 Channel 1’s volts/div or attenuator knob Set it to 0.5 volts. Make sure its
         center knob is fully clockwise. On most scopes, it’ll click at that position.
      •	 Input coupling or AC/DC/GND Set it to DC.
      •	 Channel 1’s vertical position Set it to about one-third of the way up.


     Horizontal Settings
      •	 Sweep rate or time/div Located to the right of the screen, this will probably
         be the biggest knob on the scope, and it may have a smaller knob inside, with an
         even smaller one in the center of that. Set the outer knob to 2 ms. Make sure the
         innermost knob at the center of this control is all the way clockwise.
      •	 Sweep mode Look for auto, normal and single. Set it to auto.
      •	 Horizontal display Look for a knob or buttons labeled A, A intens B, and B. Set
         it to A.
             Chapter 6       Working Your Weapons: Using Test Equipment                  79

Trigger Settings
To find the trigger controls, look for a knob labeled Level. Also look for switches for
coupling, source and slope. If your scope has separate trigger sections labeled A and
B, use A.

 •	 Source Channel 1
 •	 Coupling AC
 •	 Level Midrange; on most scopes, the knob’s indicator line will be straight up.

     These settings should result in your seeing a horizontal line across the screen. If
not, turn the Channel 1 vertical position knob back and forth. You should see the line
moving up and down.
     If you still don’t see anything, try turning up the brightness control pretty far.
Still nothing? Turn it back down to medium. If you saw a spot on the left side when
you turned it up, then your scope is not sweeping across the screen. Check that the
sweep mode is set to auto. If it’s on normal or single, you will not see anything when
no signal is applied. If you still have a blank screen, either you have made an error in
this initial setup, your scope is not getting power, or it’s not functional. Go back and
check all the settings again.


Viewing a Real Signal
Assuming you do see the line, connect a scope probe to the Channel 1 (or A) vertical
input by pressing its connector onto that channel’s input jack and then turning the
sleeve clockwise about a quarter turn until it locks. If the probe has a little 10X/1X
switch on it, set it to 10X. Usually, the switch is on the part of the probe you hold, but
some types have the switch on the connector, at the scope end. Touch your finger
to the probe’s tip and you should see about one cycle of AC on the screen. It might
wobble back and forth a little, but it should be fairly stable. If all you see is a blur, try
adjusting the trigger level knob back and forth until the image locks. You can also step
the vertical attenuator’s range up and down so the image occupies most of the screen.
You’re looking at the voltage induced into your body from nearby power wiring! Pretty
startling, isn’t it?


What All Those Knobs Do
Now that you have the scope running, let’s look at what each control does and how
to use it.

Screen Controls
Screen controls are specific to CRT displays. They adjust the electron beam for optimal
tracing, accounting for changes due to drift, writing speed, and so on. You won’t find
screen controls on scopes with LCDs.
80   How to Diagnose and Fix Everything Electronic

     Brightness or Intensity This sets the brightness of the trace. It should be adjusted
     to a medium value. Don’t crank the brightness way up for very long or you may burn
     the trace into the tube’s phosphors, and there’s no undoing that. Depending on the
     speed of the signal you’re examining, you might have to turn the brightness up so
     high that the trace will be way too bright when you remove the signal or slow the
     scope’s sweep rate back down, and you’ll have to back off the brightness again. Be
     sure to do so without much delay.
          You may find two brightness controls: one for the main sweep and another for
     the delayed sweep. The extra brightness control is there because the beam will be
     sweeping at two different speeds, and the faster sweep may be too dim to see without
     a brightness boost.
          If you turn the brightness up very high to see a fast signal, the beam’s shape may
     distort or go out of focus a bit, even if the displayed brightness remains low. That’s
     normal and is nothing to worry about, but it means the scope’s circuits are being
     driven to their maximum levels, so it’s a good idea to keep the brightness down below
     the point at which distortion becomes significant.

     Focus This focuses the beam. Turn it for the sharpest trace. When adjusting the
     astigmatism control, you may have to alternate adjusting astig and focus for maximum
     sharpness.

     Astigmatism or Astig This sets the beam’s shape and should be adjusted for
     the sharpest, thinnest trace when displaying an actual signal. You can’t set it by
     observing a flat line. An easy way to adjust it is to touch the probe’s tip to the scope’s
     cal (calibrator) terminal, which outputs a square wave signal useful for calibrating
     several of the instrument’s parameters. You can let the probe’s ground wire hang,
     since it’s already grounded to the scope through the cable. Adjust the Channel 1 input
     attenuator, the A trigger level and horizontal time/div control to get a few cycles
     of the square wave on the screen. Turn the astig control until the waveform looks
     sharpest. Normally, you won’t have to mess with it again. Now and then, you might
     touch it up when viewing very fast signals with the brightness control cranked up. If
     you do, you’ll need to reset it afterward.

     Rotation This is usually a recessed control under the screen, accessible with a
     screwdriver. It compensates for ambient magnetic fields that may cause the trace to
     be tilted. Get a flat line, use the Channel 1 vertical position knob to center it right
     down the middle, and adjust the rotation to remove tilt. Unless you take the scope to
     another locale, you’ll probably never have to touch this control again.

     Illumination This adjusts the brightness of some small incandescent bulbs around
     the edge of the screen so you can see the graticule better, especially when shooting
     photos of the screen. On a used scope, if the control does nothing, the lamps may be
     burned out. Their functionality has no effect on the operation of the scope. I always
     leave mine turned off anyway.
             Chapter 6      Working Your Weapons: Using Test Equipment                  81

Beam Finder Activating this button stops the sweep and puts a defocused blob on
the screen so you can figure out where the beam went, should it disappear. If the blob
is toward the bottom of the screen, the trace has gone off at the bottom. If it appears
near the top, the trace is above the top. If it’s in the middle, the trace is within normal
viewing limits but the horizontal sweep is not being triggered to move the beam
across the screen.
     On some scopes, the sweep continues to run but all dimensions get smaller and
everything gets brighter, so you can see a miniature version of what might otherwise
be off the screen.

Vertical Controls
The vertical controls scale the incoming signal to fit on the screen. They also allow
you to align the image to marks on the graticule for measurement purposes.

Probe Compensation This makes the probe match the input channel’s characteristics
to ensure accurate representation of incoming signals. It is also a screwdriver adjustment,
but it’s not on the scope itself. Instead, you’ll find it on the probe, and it can be at
either end. It won’t be marked, so look for a hole with a little slotted screw. Make
sure the Channel 1 input coupling is set to DC. If the probe has a 10X/1X switch,
set it to 10X. Touch the probe to the cal terminal, and adjust the input attenuator so
that the square wave uses up about two-thirds of the vertical space on the screen. Set
the time/div control so you can see between two and five cycles of the square wave.
Look at the leading edge of the waveform, where the vertical line takes a right turn
and goes horizontal at the top of each square. Adjust the probe compensation for the
squarest shape. In one direction, it’ll make a little peak that sticks up above the rest of
the waveform. In the other, it’ll round off the corner. It may not be possible to get
a perfect square, but the closer you can get, the better. See Figures 6-1 and 6-2.
     Once the probe is matched to the input channel, it’ll have to be recalibrated if you
want to use it on the other channel or on another scope. This takes only a few seconds.

Vertical Input Attenuator or Volts/Div The outer ring of this control scales the
vertical size of the incoming signal so it will fit on the screen. The voltage marking
on each range refers to how many volts it will take to move the trace up or down one




                        Figure 6-1     Probe overcompensation
82   How to Diagnose and Fix Everything Electronic




                             Figure 6-2     Probe undercompensation



     division, or box, on the graticule when you’re using a 1X probe, which passes the
     signal straight through to the scope without altering it.
          Here’s where things get interesting. Remember that 10X/1X switch on the probe?
     When set to 10X, it divides the incoming signal’s voltage by ten. If your probe has no
     switch, it still does the same thing, as long as it’s a 10X probe, which most are. You
     have to multiply the attenuator’s setting by ten to make up for that.
          For example, if you measure a voltage that makes the trace rise 3.5 divisions,
     and your attenuator is set to 0.1 volts/div, multiply 3.5 × 0.1 × 10 to get 3.5 volts.
     That might sound overly complicated, but there’s an easy way to do it: just move the
     decimal point one space to the right when looking at the attenuator. If it’s set to 0.1
     volts/div, remember it’s really reading 1 volt/div. Then multiply that by what you see
     on the screen, and you’re all set.
          Some fancier scopes automate the 10X factor when used with their own brand
     of probes. The probes alert the scope to the scaling factor, and the input attenuator’s
     markings are illuminated at the correct spot, so you don’t have to do the arithmetic.
          When you’re interpreting the displayed signal, keep in mind that the precision
     to which you can measure things depends on the setting of the attenuator. If it’s set
     to 1 volt/div (after accounting for the 10X probe factor, of course), you can visually
     estimate down to about 0.1 volt using the subdivision lines and the spaces between
     them. If it’s set to 10 volts/div, you can estimate only down to about 1 volt, since
     the same graticule box now represents 10 volts instead of 1, so each subdivision
     represents 1 volt.
          You may be wondering why probes divide the signal by 10, and why some have
     switches. To display a signal, the scope has to steal a tiny amount of it from the circuit
     you’re testing. It’s like a blood test: you have to take a little blood! The object of the
     probe’s division is to present a very high impedance (essentially, resistance) to the
     circuit to avoid loading it down—that is, stealing enough current from it to alter its
     behavior and give you a false representation of its operation. Because the 10X probe
     needs to steal only a tenth of the signal’s actual voltage, it has internal voltage-dividing
     resistors that give it an impedance of 10 MΩ, or 10 million ohms. That’s very high
             Chapter 6       Working Your Weapons: Using Test Equipment                  83

compared to the resistances used in any common circuit of the sort you’ll want to
measure. Extremely little current passes through such a high resistance, so the circuit
under test doesn’t notice it.
      If your probe offers 1X, that switch position removes the voltage divider, passing
the signal directly to the scope and resulting in an input impedance of 1 MΩ. That’s
still pretty high, but it can affect some small-signal and high-frequency circuits.
Usually, you’ll keep your probe at 10X unless the signal you want to see is so small
that you can’t get enough vertical deflection on the screen even with the attenuator
set to its most sensitive range. Now and then, you may scope a circuit that generates
enough electrical noise to get into the probe through the air, like a radio signal. When
you’re at 10X, the impedance is so high that it takes very little induced signal to
disturb your measurement. Switching to 1X may make the extraneous noise disappear,
or at least get much smaller. This kind of thing happens mostly when probing CRT TV
sets, LCD backlighting circuits and switching power supplies, all of which use high-
voltage spikes capable of radiating a significant short-range radio signal.
      Using 1X will let you see rather small signals as long as it doesn’t load them down
too much. Also, some scopes let you pull out the center knob on the attenuator to
multiply the sensitivity of the selected range by a factor of 10. Doing so causes some
signal degradation, so use this only when you really need it. You probably never will.
      Ah, that center knob. It’s called the variable attenuator. Normally, you keep it
in the fully clockwise, calibrated position so the volts/div you select will match
the graticule, allowing you to measure voltage values. Sometimes you want to do a
relative measurement—that is, one whose absolute value doesn’t matter, but you need
to know if it’s bigger or smaller than it was before, or its size relative to another signal.
To make such measurements easy to read, it’s very helpful to line up both ends of the
signal with lines on the graticule. Turning the center knob counterclockwise gradually
increases the attenuation, reducing the vertical size of the signal and letting you align
its top and bottom with whatever you like. It’s crucial to remember, though, that you
can’t take an actual voltage measurement this way; the vertical spread of the signal
has no absolute meaning whenever the variable attenuator is engaged. To remind you,
many scopes have a little “uncal” light near the attenuator so you’ll know when the
variable attenuation is on and the channel is uncalibrated.

Input Coupling (AC/DC/GND) This determines how the signal is coupled, or
transferred, into the vertical amplifiers, and it’s one of the most important options
on a scope. As you will see, the choice of coupling enables a neat trick for examining
signal details and is not limited to being used in the obvious way, with DC for DC
signals and AC for AC signals. You’ll find yourself switching between the two settings
quite often when exploring many types of signals.

GND The GND setting simply grounds the input of the scope, permitting no voltage
or signal from the probe to enter, and discharging the coupling capacitor used in the
AC setting. (More about that shortly.) It does not ground the probe tip! You don’t need
to remove the probe from the circuit under test to switch to the GND setting. This
selection is used to position the trace at a desired reference point on the screen, using
84   How to Diagnose and Fix Everything Electronic

     the input channel’s vertical position control, with no influence from incoming signals.
     Not all scopes have a GND setting. Some probes offer it on their 10X/1X switches. If
     you don’t have one in either place, it’s no big deal, because you can always touch the
     probe tip to the ground clip. Having a GND setting just makes getting a clean, straight
     line of 0 volts a bit more convenient.

     DC In the DC position, the signal is directly coupled, and whatever DC voltage is
     present will be plotted on the screen. If you want to measure the voltage of a power
     supply or the bias voltage on a transistor, use this position.
          To measure a DC voltage, first remove the probe from the circuit and touch it to
     the ground clip, or switch to the GND setting, and turn the vertical position knob to
     wherever on the screen you want to call 0 volts. If you’re measuring positive voltage,
     the bottommost graticule line is a good place to put the trace. If the voltage is negative,
     set the trace at the topmost line, because it will move down when the signal is applied.
     Then touch the probe to the point you want to measure, or switch back from GND to
     DC, and observe how many graticule divisions the trace rises or falls. Don’t forget to
     multiply the attenuator’s marking by ten to compensate for your 10X probe!
          If you touch a voltage point with the probe and the trace disappears, you have
     probably driven it off the screen with a voltage bigger than can be handled by the range
     you’ve selected with the vertical attenuator control, so set that to higher voltages per
     division until you can see the trace. After switching the attenuator to a different range,
     perform the zero setting again before trying to estimate a measurement from the
     screen, because sometimes it drifts a little bit when you change the range.

     AC Many signals contain both DC and AC components. They have a DC voltage offset
     from 0 volts, but they’re also not just a straight line; there’s variation in the voltage
     level, representing information or noise. So how is a varying DC voltage AC? Isn’t it
     one or the other?

         As I mentioned in Chapter 5, voltage level and polarity are entirely relative. Any
     voltage can be positive with respect to one point and negative with respect to another.




                  +12V                                                   AC
                                                                         Component




                                                  DC Component



                    OV

                      –V

                    Figure 6-3     The AC and DC components of a signal
             Chapter 6      Working Your Weapons: Using Test Equipment                 85

Take a look at Figure 6-3. That signal is above ground in the positive direction, with
none of it going negative, so it’s a DC signal with respect to ground, and its height
above ground is its DC offset or DC component. The top of it, however, is wiggling up
and down. Imagine if you could block the DC offset, and that dotted line became
0 volts. The signal would then be AC, with half above the line and half below.
     It looks nice on paper, but you can’t actually do that, right? Sure you can! Passing
the signal through a capacitor will block the DC level, but, as the signal wiggles, those
changes will get through, resulting in a true AC signal swinging above and below
ground, with polarity going positive and negative. Hence, those wiggles are known
as the AC component of the DC voltage. (And now you know a quick-and-dirty way
to turn a positive voltage into a negative one! See, I wasn’t kidding, there truly is no
absolute polarity.)
     When you select AC coupling, the scope inserts a coupling capacitor between the
probe and the vertical amplifier, blocking any DC voltage from deflecting the beam.
This changes everything! By blocking the DC component of a signal and passing only
the AC component, you can examine that component in great detail. Let’s see how.
     Suppose you have a 12-volt power supply in an LCD monitor with erratic
operation. The backlight doesn’t like to turn on. When it finally does, sometimes it
shuts itself off. You suspect the supply might not be putting out clean power. In other
words, some noise could be riding on top of the voltage, perhaps caused by weak filter
capacitors, confusing the microprocessor and turning it off. You fire up your scope,
set it to DC coupling and check that 12-volt line, but it looks okay. Hmmm…there
might be a little blurriness on the line, but it’s hard to tell for sure, so you crank up
the sensitivity of the vertical attenuator to take a closer look. Oops! The trace is now
off the screen. You turn the vertical position control down to get it back, but now that
control is as low as it will go. You can see the line, but you can’t check for any small
spikes or wobbles in it because it keeps going off the screen every time you up the
sensitivity enough to examine the small stuff.
     If only that same noise were riding on 0 volts, instead of 12, then the trace would
stay put and you could fill the screen with even the tiniest changes. No problem.
Switch to AC coupling to block the DC component of the signal and you can crank the
attenuator for maximum sensitivity without budging the trace. It’s a very powerful
technique for examining signals with both DC and AC components. Many signals are
of that form, and you’ll use AC coupling quite often, regardless of whether the signal
is really AC or not. In fact, a true AC signal (one that swings positive and negative
with respect to ground, with no DC offset) will read exactly the same with either DC
or AC coupling, so switching between the two is an easy way to see if an offset exists.
If the trace doesn’t shift vertically when you flip the coupling switch, there’s no offset.
     If the signal’s changes are slow enough, the coupling capacitor charges up and
eventually begins to block the slowly changing signal voltage until the other half
of the cycle discharges it. This is called low-frequency rolloff; the cap acts as a high-
pass filter, permitting high frequencies to pass through, while gradually rolling off
(attenuating) lower ones, passing nothing when the frequency reaches zero. The
effect will distort low-frequency signals, causing their flat areas to droop as the cap
charges. See Figure 6-4.
86   How to Diagnose and Fix Everything Electronic




                             Figure 6-4     Tilt due to low-frequency
                             rolloff



          To see this in action, view about four cycles of the square wave from the cal
     terminal and switch between AC and DC coupling. (Adjust the vertical position if a
     DC offset moves part of the waveform off the screen when you change the coupling.)
     The flat tops and bottoms look tilted with AC coupling, losing amplitude and heading
     toward the middle of the waveform as the cap charges. As the signal frequency rises,
     the cap has less time to charge, so this effect fades away, for a truer representation
     of the signal. When you’re viewing low-frequency signals with AC coupling, always
     keep in mind that long, sloping areas may in fact be flat, and the scope’s coupling
     cap might be causing the slope. The easy way to verify the presence of rolloff is to
     switch to DC coupling (unless a DC offset drives the trace too far off the screen for the
     vertical position control’s range to bring it back). If the sloping lines become flat, you
     know the cap was fooling you.
          After using AC coupling, always ground the probe tip or momentarily switch the
     input coupler to GND to discharge the coupling capacitor before probing another
     point. Otherwise, whatever is stored on it will discharge into the next point you touch,
     confusing your reading and possibly damaging the circuit under test, although there’s
     only a remote chance of that. When you ground the probe, you should see the trace
     jump for a fraction of a second and then return to where it was as the cap discharges.
          AC coupling can be inconvenient when you’re working with signals that change
     amplitude (vertical size) in an asymmetrical fashion. Analog video, for instance, is
     “clamped” to a fixed voltage, with its sync pulses not deviating from their position
     at the bottom as the video information at the top changes with the TV picture’s
     content. Using AC coupling with such a signal will cause both ends of the waveform to
     bounce around as video content rises and falls, because the midpoint of the signal is
     constantly shifting. The effect is disconcerting and difficult to interpret. Video signals
     and others with similar asymmetry, like digital pulse streams, are best viewed with
     DC coupling, so that the unchanging end of the signal stays put.
          The example was from a real case of an LCD monitor I fixed using the AC coupling
     on a DC signal technique. Sure enough, there were narrow, 1/2-volt spikes on the
             Chapter 6      Working Your Weapons: Using Test Equipment                  87

power supply line confusing the microprocessor and tripping the unit off. Once I
switched to AC coupling and saw the spikes on the filter capacitor’s positive terminal,
I didn’t even have to check the cap, because I knew a good one would have filtered
them out. I just popped in a new cap. A quick check confirmed that the spikes were
gone, and the noise on that line was now well under 100 mv. The micro was happy
and so was I. The monitor worked great. Without the AC coupling trick, I’d never have
known those spikes were there.

Vertical Position This moves the trace up and down. Use it to set the zero reference
point when taking DC measurements. Otherwise, set it wherever is best for examining
the signal you’re viewing. When using the scope in dual-trace mode, set the two input
channels’ vertical position controls to keep the waveforms separated. It’s conventional
to put channel one in the top half of the screen and channel two in the bottom, but
you can place them wherever you want, even on top of each other.

Bandwidth Limit This limits the frequency response of the vertical amplifiers.
In 100-MHz scopes, the limit is usually 20 MHz. Switching in the limit removes noise
bleeding in from external sources, particularly FM radio stations. With 100-MHz
bandwidth, the scope can pick up the lower two-thirds of the FM band, resulting in
a noisy-looking signal if you happen to live near an FM broadcast station. Some other
radio services may get into your measurements too, if they’re strong enough. Leave
the bandwidth limit turned off unless noise problems are making your trace blurry.

Vertical Mode On a dual-trace scope, this selects which channels you will view, and
how. Most scopes offer these options:

 •	 Ch 1 You will see only channel 1. Input from channel 2 can still be used to feed
    the trigger if you want, even though you can’t see it.
 •	 Ch 2 The same, but in reverse. You’ll see only channel 2.
 •	 Add The voltages of the two channels will be added together and shown as
    one trace.

     Adding two signals together is pretty pointless, so why is this here? One or more
of your channels should have a button marked invert or inv. Pressing it makes the
channel flip the waveform passing through it upside down, with positive voltages
deflecting downward and negative ones upward. If you flip one channel upside down
and then select the add mode, the signals will be subtracted, and that is very useful in
certain circumstances.
     Let’s say you have an audio amplifier with some distortion. Or, perhaps, the high
frequency response is poor. You want to find which stages are causing the problem.
Feed some audio to the amplifier’s input. Set both scope channels to AC coupling.
Connect one channel to a stage’s input and the other to the stage’s output. If the
amplifier hasn’t already inverted the signal, do the invert-add trick. Then use the
vertical attenuator of the channel with the bigger signal (usually the stage’s output) to
reduce its displayed amplitude until the signals cancel out and the resulting trace is
as flat as possible. What’s left is the difference between the two signals: the distortion.
88   How to Diagnose and Fix Everything Electronic

     Using this technique, you can easily see what a circuit is doing to a signal, and the
     results may be enlightening, leading you to a diagnosis. Most amplifier stages will
     show some difference, but it should be minor. When you see something significant,
     you’ve found the errant stage.

     Chop This puts the scope into dual-trace mode, displaying two signals at once.
     The two will appear to be independent, and you can set the vertical attenuation and
     position of each channel at will. The horizontal sweep speed set with the time/div
     control will apply to both channels, and the scope will trigger and start the sweep
     based on the timing of whichever of the two channels you choose for triggering.
     However, the scope is not truly displaying two simultaneous events; it just looks
     that way. In reality, it is rapidly switching between the two channels, with the beam
     bouncing between them, drawing a little bit of one channel and then a little bit of the
     other. It is chopping them up.
          This method works very well, ensuring that the time relationship between the
     two signals is well preserved, since they are really being made by the same beam as
     it traverses the screen from left to right. It has a few serious limitations, however.
     If you crank the sweep speed way up, you can see the alternating segments of the
     two channels and the gaps between them. So, chop mode is not useful at high sweep
     speeds. Also, if the two signals are harmonically unrelated (their frequencies are not a
     simple ratio, so the cycles don’t coincide), only the channel chosen for triggering will
     be visible; the other will be a blur.

     Alternate or Alt This is the other dual-trace mode, and it works rather differently.
     Instead of chopping the waveform, it draws an entire channel and then goes back
     and draws the other one, in two separate, alternating sweeps. Alt mode leaves no
     gaps in the traces, so it’s suitable even for very fast sweep speeds. Plus, under certain
     circumstances, you can view harmonically unrelated signals, with separate triggering
     for each channel making them both look stable.
          Alt mode has its own limitations, though. At slow sweep speeds, you’ll see the
     two sweeps occur, one after the other, with the previous one fading away, resulting in
     a rather uncomfortable blinking or flashing effect. Also—and this one is much more
     serious—the timing relationship between the two signals may be disturbed, because
     of when the scope triggers and how long it takes to sweep the screen before it has a
     chance to draw the second waveform. In alt mode, the displayed alignment in time
     of one channel to the other cannot be trusted. Always choose chop mode when you
     need to compare or align the timing of two signals. In fact, use chop mode as much as
     possible, and select alt mode only when chopping interferes with the waveform or the
     two signals you’re viewing are unrelated in time, so each one needs its own trigger.

     X-Y This mode stops the sweep and lets you drive the horizontal deflection from a
     signal input to channel 2. Be careful when pressing this button, because the stopped
     beam will create a very bright spot on the screen and can quickly burn the tube’s
     phosphors. Turn the brightness all the way down before trying it, and then gradually
     turn it up until you see the spot.
             Chapter 6      Working Your Weapons: Using Test Equipment                 89

    At one time, X-Y mode was a useful way of detecting nonlinearity, measuring
frequency and some other parameters. Today, it has little or no application, at least
not for general service work. I have never needed to use X-Y mode even a single time.

Trigger Controls
To present a stable waveform, the scope must trigger, or begin each sweep, at the same
point in each cycle of the incoming signal, so the beam will draw the same signal
features on top of the last ones. Otherwise, all you’ll see is a blur. Stable triggering is
one of the most critical features on a scope, and it’s important that you get good at
using the trigger controls to achieve it.

Trigger Lock Light This indicator tells you when the trigger is locked to a feature
of the signal. When it’s on, you should see a stable waveform. If not, some other
control is improperly set, or the trigger may be locking to more than one spot in each
cycle of the waveform.

Source This selects which channel will be used to trigger the sweep, along with
some other options:

 •	 Ch 1 Channel 1’s signal will feed the trigger. Use this mode for single-channel
    operation or for dual-channel work when you want channel 1’s signal to control
    timing.
 •	 Ch 2 Channel 2’s signal will feed the trigger.
 •	 Alt In alt mode, each channel will feed the trigger, one after another. This
    completely invalidates the timing relationship between displayed signals, because
    you have no idea how much time has elapsed between when the first channel’s
    sweep finished and when the second channel’s sweep began. It’s useful when you
    want to look at two signals whose periods are not related, and you want them both
    to display stably. Just remember that it’s like having two separate scopes; no time
    relationship exists between the displayed signals.
 •	 Line This uses the 60-Hz AC line as a timing reference, generating 60 sweeps
    per second. It’s useful when viewing signals at or very near that frequency whose
    own features make for difficult triggering. Now and then, it can be handy when
    troubleshooting line-operated gear, especially linear power supplies.
 •	 External or ext Many scopes have extra inputs that can be used as vertical
    channels and/or trigger inputs. External triggering is great for locking very
    complex signals the normal trigger can’t get a grip on.
    For instance, when adjusting VCR tape paths while viewing the RF (radio-frequency)
    waveform from the video heads, there is no stable way to trigger at the start of each
    head’s sweep across the tape, using the signal it produces. Instead, you must drive
    the scope trigger from another signal in the VCR that is synced to the headwheel
    rotation. The external trigger input provides a place to feed it in, keeping channel 2
    (from which you could accomplish the same thing) free for viewing other signals.
90   How to Diagnose and Fix Everything Electronic

     Coupling This is somewhat like the input coupling on the vertical amplifiers, but it
     offers a few more choices specific to triggering needs:

      •	 DC The trigger will lock to a specific signal voltage relative to ground. It’s most
         useful with asymmetrical waveforms.
      •	 AC The trigger will lock to a voltage above or below the midpoint of the signal,
         regardless of its voltage relative to ground. It works just like the AC coupling option
         on the vertical amplifiers, placing a coupling capacitor in line to block the signal’s
         DC component. You can use AC trigger coupling even when you use DC coupling
         for the vertical channel. Most of the time, you’ll use this mode because it makes
         triggering easy as you look at various signals with different DC components.
      •	 HF reject This feeds the signal through a low-pass filter, smoothing out high-
         frequency noise or signal features that might confuse the trigger and cause it to
         trip where you don’t want it to. If your waveform has high-frequency components
         causing jittery display, try this option. If you want to trigger on a high-frequency
         feature, however, selecting HF reject will prevent triggering. Using this setting
         affects only the trigger operation; the signal going to the vertical amplifier is not
         filtered.
      •	 LF reject This feeds the signal through a high-pass filter, rolling off low frequencies
         and preventing them from tripping the trigger. Use it to help trigger on high-
         frequency signal components when lower-frequency elements are causing
         triggering where you don’t want it. Again, the filtering affects only the trigger.
      •	 TV-H This is a specialized trigger mode for use with analog TV signals. It comes
         from a time when much service work was on TV sets, and not all scopes offer it. It
         is optimized to help the scope trigger on the horizontal sync pulses in the TV signal.
      •	 TV-V This enables triggering on the vertical sync pulses in a TV signal.

     Slope This selects whether the trigger locks on signal features that are rising or
     falling. With many kinds of signals, like audio and oscillators, it doesn’t matter.
     Normally, leave it on +. When you want to trigger on the falling edge of a waveform,
     switch it to –. To see the slope feature in action, connect the probe to the cal terminal,
     get a locked waveform and then switch between the slopes. Look at the leftmost edge
     of the screen to see what the waveform was doing when the sweep triggered.

     Level This sets the voltage level at which the trigger will trip. It has no calibration.
     Just turn it back and forth until the trigger locks. If the trigger stays locked through a
     wide swath of this control’s range, that indicates a solid trigger lock, and you should
     see a very stable display. If you can get trigger lock only over a very narrow range
     of the level control, the lock is not great, and you can expect the waveform to jump
     around if the signal level or shape changes even a little bit. To get a better grip on the
     signal, try the various coupling options, especially HF reject and LF reject, rotating
     the level control back and forth for each one.

     Holdoff This keeps the trigger held off, or unable to trip, for an adjustable period of
     time after its last trigger event. It’s used on complex signals with irregularly spaced
     features of similar voltages, to avoid having more than one in each cycle trip the
             Chapter 6      Working Your Weapons: Using Test Equipment                  91

trigger and blur the displayed waveform. If you can’t get a waveform to stabilize any
other way, try turning this control. Otherwise, keep it at the “normal” position, which
reduces holdoff to a minimum. Forgetting and leaving it on can make triggering
confusingly difficult, as the holdoff will make the trigger miss consecutive cycles if it’s
set for too long a period, resulting in a jumpy mess.

Horizontal Controls
The horizontal axis represents time, and the scope’s drawing of the waveform from
left to right is called the sweep. The settings controlling it determine what you’ll see,
even more than do those for the vertical parameters.

Horizontal Position This positions the trace left and right. Set it to fill the screen,
with the left edge of the trace just off the left side. Now and then you may wish to
line up a signal feature with the graticule to make a rough measurement of period or
frequency, and you can use this control to do so.

Sweep Mode This control offers three options: auto, normal and single.

 •	 Auto The trigger will lock to the incoming signal, and sweep will begin. When
    there’s no signal, or the trigger isn’t locking on it for some reason, the sweep will
    go into free-run mode, triggering itself continuously so you can see a flat line, or, in
    the case of trigger unlock, a blur. This is the mode you will use most of the time.
 •	 Normal Sweep will begin when the trigger locks on a signal but will stop when
    the signal ceases. This can be handy for observing when rapid interruptions occur
    in intermittent signals, because the trace will blink at the moment the signal
    disappears. It’s a little disconcerting sometimes, though, because if the screen
    goes blank, you don’t know why. Maybe the trace is off the screen, maybe the
    brightness is too low, or maybe the trigger isn’t locked.
 •	 Single This is for single-sweep mode, in which the trigger will initiate one
    sweep and then halt until you press the reset button (which may be the single
    button itself). It helps you determine when a signal has occurred, because you’ll
    see the flash of one sweep go by. Look for a little indicator light near the single
    button labeled Ready or Armed. When it’s lit, the sweep can be fired one time.
    After that firing, the light will go out, and you must press the reset button to
    rearm the sweep. You won’t use this mode very often.

Time/Div The outer ring of this large control sets the timebase, or sweep speed at
which the beam will travel across the screen from left to right. It is calibrated by time
in seconds, milliseconds (ms, or thousandths of a second) and microseconds (µs, or
millionths of a second). The calibration number refers to how long the beam will take
to traverse one division, or graticule box.
     For a signal of a given frequency, the faster you set the sweep, the more horizontally
spread the display will be, and the fewer cycles of the signal you will see at one time.
Often, you will want to set it as fast as possible to see the most detail, but not always.
In some instances, the aggregate effect of many displayed signal cycles can be more
revealing than is a singular signal feature. If you have low-frequency variation, such
92   How to Diagnose and Fix Everything Electronic

     as AC line hum, on a fairly fast signal, you can’t see the hum if the sweep is set fast
     enough to see individual cycles of the signal. But when you turn the sweep speed low
     enough to view a 60-cycle event, you will see the hum clearly, even though the signal
     itself will be too crammed together to be resolvable.
          To see the effects of various sweep rates, look at the calibrator’s square wave and
     click the time/div knob through its ranges. On most scopes, the number of displayed
     waves will grow or shrink with the sweep rate. If it doesn’t, your scope is changing the
     frequency of the calibrator to match the timebase. Some of them do that.

     Variable Time The center knob uncalibrates the timebase, slowing it down as you
     turn the knob counterclockwise. This is the horizontal equivalent of the vertical
     channels’ variable attenuators, and it can be useful for lining events up with the
     graticule during relative time measurements of two signals. Normally, you’ll leave it
     in the fully clockwise position. As with the variable attenuators, it has an “uncal” light
     to remind you that the timebase no longer matches the graticule.

     Pull X10 On most scopes, pulling out the variable time knob speeds up the sweep
     rate by a factor of ten. The sweep does remain calibrated in this mode. It’s a quick-
     and-dirty way to spread out a signal, and it also lets you get to the very fastest sweep
     rate by turning the time/div knob all the way up and then pulling this one out.
     Normally, keep this knob pushed in, and pull it only when you really need it. Don’t
     forget to push it in again, or your time measurements will be off by a factor of ten.

     Delayed Sweep Controls
     Delayed sweep is an advanced scope function you won’t need for basic repair work,
     but it’s worth learning for those more complex situations, like servicing camcorder
     motor control servos, in which it’s essential. My apologies for any neck injuries caused
     by making your head spin while reading this section! Once you actually play with
     delayed sweep a few times, you’ll discover it’s really not difficult, and it’s quite nifty.
           With delayed sweep, your scope becomes a magnifying glass, allowing you to
     zoom in on any signal feature, even though it’s not the one on which you’re triggering
     the main sweep. Why do this? It’s very powerful, providing a level of signal detail you
     couldn’t otherwise examine.
           Let’s say you have a sine wave from an oscillator, but it doesn’t look quite right.
     A spike or something is distorting its shape at a particular spot. It’s hard to tell what
     it is, but you can see a thickening of the trace at that point. You want to get up close
     and personal with that spot so you can really see the details of the distortion and
     determine what’s causing it. You scope the sine wave and crank up the sweep rate, but
     when you get it going fast enough to spread out the mystery spot, it has already gone
     off the right side of the screen. You turn the sweep rate down a little bit, but now the
     signal is too crammed together to permit a good look at the spot.
           Cue superhero music. This is a job for…Delayed Sweep, Slayer of Stubborn
     Signals, Vanquisher of Villainous Voltages! In this mode, the scope triggers on the
     waveform as usual, as set by the A trigger. It begins sweeping at the rate selected with
     the main time/div knob. After a period of time you set with the delay time multiplier
             Chapter 6      Working Your Weapons: Using Test Equipment                 93

knob, the B timebase takes over and the beam finishes the sweep at the speed set by
that timebase’s knob, the one inside the main sweep’s knob.
    The result is a compound view of the signal, with a lower sweep rate for the
events leading up to your spot of interest, followed by a stretched-out, detailed view of
the spot! Even cooler, you can rotate the delay time multiplier knob and scan through
the entire signal, examining any part of it. It’s practically a CAT scan for circuitry, and
you don’t even need a litter box. Let’s look at the controls involved with setting up the
delayed sweep mode.

Delay Time or B Timebase This control, located inside the main time/div knob,
sets the speed of the B sweep, which stretches out the waveform for close examination.
Think of it like a zoom lens on a camera: the faster you set it, the more you’re zooming
in for a closer look at a smaller area.
     On some scopes, notably those made by Tektronix, the same knob is used for
both the A and B sweeps. To engage the B sweep, you pull out the knob, mechanically
separating the two timebase controls. In this position, the outer ring, which sets the A
sweep rate, will not move when you turn the knob clockwise to speed up the B sweep.

Horizontal Display Mode This selects which timebase (sweep generator) will drive
the beam across the screen, and is how you choose between normal and delayed
sweep modes.

 •	 A The main timebase, which is set by the big time/div control, will control the
    sweep. Delayed sweep mode will not be engaged.
 •	 B The secondary timebase, set by the smaller control inside the time/div knob,
    will control the sweep. This may be set equal to or faster than the main timebase,
    but not slower.
 •	 Alt Both timebases will be displayed, one on top of the other. The A sweep’s
    display will be highlighted over the area that the B sweep is stretching out. The
    faster you make the B sweep rate, the narrower the highlight will get, and the
    more stretched the displayed B sweep will be. See Figure 6-5. Look for a knob
    called trace separation. It lets you position the B sweep’s display above or below
    that of the A sweep, so you can see them better. It has no calibration or meaning
    in voltage measurement terms; it’s just a convenience.
    If your scope has two brightness controls, you may need to increase the
    brightness of the B sweep to keep it visible at high sweep rates. The faster the
    beam sweeps, the dimmer it will appear.
 •	 A intens B This stands for “A intensified by B” and shows you the A sweep and the
    highlight of the portion of the signal the B sweep will cover, just as is shown in Alt
    mode. It’s useful for zeroing in on the spot you wish to examine without cluttering
    up the screen with the B trace, before you switch to a mode that displays B.
    In both Alt and A intens B modes, the highlighting of the A sweep may not be
    visible if you have the A brightness control set too high. Try turning it down for
    a more prominent highlight.
94   How to Diagnose and Fix Everything Electronic



                 A sweep
                                                                           Highlighted
                                                                           area




         Magnified view of
         highlighted area
         using B sweep



                             Figure 6-5    Alt mode delayed sweep display of
                             the rising edge of the scope’s calibrator square
                             wave. The A trace is on the top, with the B trace
                             below it. Notice the highlighting of the portion of
                             the waveform being stretched out by the B trace.


      •	 B This shows only the B trace, at the sweep rate selected by the B or delay time
         setting (the knob inside the big, outer time/div one). It turns off the A trace, but
         the settings which result in the delayed sweep B view remain in effect, including
         triggering, A sweep rate and delay time. Once you’ve zoomed in on your spot
         of interest, you can switch to this setting to see only the detailed area, without
         the main waveform from which it is derived. In this mode, the trace separation
         control doesn’t do anything.
      •	 Start after delay/triggered This determines what happens after the A sweep
         has reached the beginning of the intensified area where it will switch to B. In
         start after delay mode, the B sweep begins as soon as the A sweep hits that point.
         This lets you scan continuously through the signal with the delay time multiplier
         control, and is my preferred way of using delayed sweep. It has one drawback,
         though. Any jitter (instability) in the triggering of the A sweep will get magnified
         by the faster sweep rate of the B sweep, resulting in the stretched waveform’s
         wobbling back and forth. Sometimes the wobble is bad enough to make examining
         the signal difficult.
         To remove the wobble, select the start triggered mode. Then, when the A sweep
         reaches the intensified portion, the B sweep will wait for a trigger before
         beginning. This is what those B trigger controls are for. What’s great about having
         a separate trigger for this function is that you don’t have to use the same trigger
         settings for the B sweep that you selected for the main trigger. So, you can treat
         the detailed area as a separate signal, setting the trigger for best operation on its
         particular features.
             Chapter 6      Working Your Weapons: Using Test Equipment                 95

    The downside to this mode is that you cannot scroll smoothly through a signal,
    because the B sweep waits for a trigger. You can see only features on which it has
    triggered. Keep your scope in start after delay mode unless you run into wobble
    problems or are examining fine details in a complex signal, like an analog video
    signal, with many subparts.

Delay Time Multiplier This sets how many horizontal divisions the delay will
wait before starting the B sweep (or looking for a new trigger for it, in the start
triggered mode), and is referenced to the A sweep rate. For example, if the A sweep
is set to 0.1 ms and the delay time multiplier knob is at 4.5, it’ll wait 0.45 ms before
the highlighted area begins. For most work, you can ignore the numbers and just
keep turning the knob to scan through the signal, using the highlight to pick your
expansion area. Most scopes provide a vernier (gear reduction) knob for fine control.

B Ends A This turns off the A sweep after the B sweep starts. On some scopes, it
presents both sweeps in one line, switching from A to B when B begins. On others, the
two sweeps are still on separate lines, and A just disappears after the point where B
takes over.

Try It!
How are those neck muscles doing? Ready to try some of this? Let’s use delayed
sweep to get a close-up look at the rising edge of your scope’s calibrator square wave.
     Connect the probe to the cal terminal and set up the scope for normal, undelayed
operation by selecting the A horizontal display mode. Adjust the vertical, trigger and
time/div controls to see a couple of cycles on the screen, and put them on the top half.
     Disengage B ends A mode if it is currently turned on. Make sure start after delay
is engaged, not start triggered. Select the A intens B mode and move the delay time
multiplier knob to center the highlight over the leading (rising) edge of the square wave,
with the start of the highlight just before (to the left of) the edge. Set the brightness
controls as desired so you can see the highlight clearly. The A brightness control will
adjust the bulk of the waveform, with the B brightness setting the highlight’s intensity.
     If more than one cycle of the waveform is on the screen, any of the leading edges
will do. It’s best to keep the A sweep as fast as possible, though, without losing the
feature you want to examine off the right side, so try not to have more than one or
two cycles visible.
     Adjust the delay time (B sweep rate) so that the highlight is just a little wider than
the edge, covering a smidge of the waveform before and after it. Note that the faster
you make the B sweep, the narrower the highlight gets, meaning you will be zooming
in on a smaller area, enlarging it proportionally more.
     Now switch to alt mode. You should see the zoomed-in rising edge of the
waveform superimposed on the original wave. Use the trace separator knob to move
it down so you can see it clearly. Increase the B sweep rate one step at a time. As you
crank it up, the magnified edge will move off the right side of the screen, and you’ll
have to turn the delay time multiplier knob clockwise just a tad to bring it back. If the
magnified waveform gets too dim, crank up the B brightness.
96   How to Diagnose and Fix Everything Electronic

          When you get the B sweep going pretty fast, you can clearly see the edge’s slope, and
     you may also notice some wobbles or other minuscule features at its bottom and top,
     details you can’t see at all without the magic of delayed sweep! For fun, scan through
     the waveform with the delay time multiplier knob, and take a look at the falling edge
     too. Pretty awesome, isn’t it? Enjoy! Just don’t forget to turn the A brightness down
     when you switch back to normal, undelayed operation, if you turned it up.

     Cursor Controls
     If your scope offers numerical calculation, it will have cursor controls that let you specify
     the parts of the waveform you wish to measure. The results of the measurements will be
     shown as numbers on the screen, along with the waveforms.
          The layout of controls can vary quite a bit in this department, but the principles
     are pretty universal. For amplitude measurements, you move two horizontal cursors up
     and down to read the voltage difference between them. For time measurements, you
     move two vertical cursors left and right to measure the time difference between them,
     or to calculate approximate frequency. See Figure 6-6.
          On some scopes, you can lock one cursor to the other after you’ve set them, so
     you can move one to the start of a waveform feature and the other will follow, letting
     you see how the signal aligns against the second one.
          Always keep in mind that these measurements provide nowhere near the
     accuracy or precision of those you’ll get from your DMM or frequency counter! Still,
     you can’t measure the voltage or frequency of items within signals with anything but
     a scope.


     Digital Differences
     Operating a digital scope isn’t that much different from using an analog, but there are
     some items to keep in mind.




                              Figure 6-6    Cursors and frequency
                              (1/time) measurement on a Tektronix
                              2445 analog scope
             Chapter 6      Working Your Weapons: Using Test Equipment                 97

     First, many of the controls on an analog instrument are replaced by menus on its
digital counterpart. This approach unclutters the front panel, but it’s slower and more
awkward to have to step through nested menus than it is to reach for a knob. As long
as you keep the basic functions of vertical, horizontal and triggering in mind, though,
you should have no trouble remembering where to find the options you need.
     All the screen controls are gone. You don’t need astigmatism, focus, illumination
and separate A and B brightness, because the display is an LCD screen and it is not
being swept at varying rates by a beam. You’ll find a main brightness or contrast
adjustment in a menu. Once set, it won’t require any changes for different modes or
sweep speeds.
     The display will show various operating parameters like trigger status, volts/div
and time/div, so you don’t have to take your eyes off the screen and interpret a bunch
of controls.
     Digitals are generally more accurate, especially in the horizontal (time) domain.
They also tend to have more stable triggering and very little drift.
     Delayed sweep may be handled a bit differently. On the Tektronix TDS-220, for
instance, the equivalent of the analog scope’s highlight is called the Window Zone, and
you set the delay time multiplier and window width using the horizontal position and
time knobs after selecting the mode from a menu. Instead of a highlight, you get a set
of long, vertical cursors. Then you select Window to see the magnified waveform. The
principles are the same, of course, but there’s no equivalent to the analog instrument’s
alt mode, in which both the undelayed and delayed sweeps are shown simultaneously.
     More than likely, the digital scope will include various acquisition modes, so you
can grab waveforms, store them and display them along with live signals. It’ll probably
also have measurement options for frequency, period, peak-to-peak voltage, RMS
voltage and so on. Especially nice is the auto setup function, which sets the vertical,
horizontal and trigger for proper display of a cycle or two of most waveforms, without
your having to twist a single knob. Hook up the probe, hit the auto button and there’s
your signal. It’s the oscilloscope equivalent of autoranging on a DMM, and it saves you
a lot of time and effort.
     Reading the screen on a digital scope requires more interpretation. The limitations
imposed by the digital sampling process, the finite resolution of the dot-matrix display
and the slower-than-real-time screen updating have to be kept in mind at all times. For
one thing, curved areas can have jagged edges, and it’s important to remember that
they are not really in the signal. Also, lines may appear thicker and noisier than they
really are, due to digital sampling noise. Aliasing of the signal against the sampling rate
and also against the screen resolution can seriously misrepresent waveforms under
certain circumstances, as discussed in Chapter 2. Finally, the slow updating means
some details get missed. All scopes, analog or digital, show you snapshots, but with
digitals there’s much more time between snaps.
     When the input signal disappears, many digital scopes freeze the waveform on the
screen for a moment before the auto sweep kicks in and replaces it with a flat line.
This makes it hard to know exactly when signals stop. If you’re wiggling a board while
watching for an intermittent, the time lag can hinder your efforts to locate the source
of the dropout.
98   How to Diagnose and Fix Everything Electronic

          Overall, a digital instrument offers more stability, options and conveniences,
     but an analog scope gives you a truer representation. As with just about everything,
     digital is the future, like it or not, and analog scopes will probably disappear from the
     marketplace in the next few years. If you snagged a good one, hang onto it! If you
     chose a digital instrument, just keep these caveats in mind and you’ll be fine.



Soldering Iron
     Soldering is probably the most frequent task you’ll perform in repair work. It’s also
     one of the easiest ways to do damage. Competent soldering technique is essential, so
     let’s look at how to do it.
          Never solder with power applied to the board! The potential for causing calamity
     is tremendous. First, you may create a path from the joint, through your iron to
     ground via the house wiring, resulting in unwanted current. Second, it’s very easy for
     the iron’s tip to slip off the joint and touch other items nearby. Make sure power is
     truly disconnected, remembering that many products don’t actually remove all power
     with the on/off switch. Unplug the item or remove the batteries to be sure.
          A good solder joint is a molecular bond, not just a slapping of some molten metal
     on the surface. The solder actually flows into the metal of the component’s leads and
     the copper circuit board traces. When it doesn’t, the result is called a cold solder joint,
     and it will fail fairly quickly, developing resistance or, in some cases, completely
     stopping the passage of current.
          To get a good joint, first tin the iron’s tip. Warm up the iron to its full temperature
     and then feed a little bit of solder onto the tip. It should melt readily; if not, the tip
     isn’t hot enough. Coat the tip with solder—don’t overdo it—and then wipe the tip on
     the moistened sponge in the iron’s base. If you have no sponge, you can use a damp
     (not dripping wet) paper towel, but strictly avoid wiping the tip on anything plastic.
     Melted plastic contaminates the tip badly and is tough to remove.
          Once the tip is nice and shiny, put another small drop of solder on it. Press the tip
     onto the work to be soldered, being sure it makes contact with both the circuit board’s
     pad and the component’s lead (or contact point, in the case of surface-mounted, leadless
     parts). Then feed some solder into the space where the lead and the pad meet, until you
     have enough melted solder to cover both without creating a big blob. See Figure 6-7. To
     get a good idea of how much solder to use, look at the other pads. See Figure 6-8.
          As the solder feeds, it should flow into the metal. Check around the edges for
     smooth integration into the joint. If you see a ring of brown rosin, gently scrape it
     away with an X-Acto knife or very small screwdriver so you can get a good look. Also
     check for proper flow around the component lead. Sometimes the flow is fine to the
     board’s pad, but the solder is pooled around the lead without having bonded to it
     because the iron’s tip didn’t make good enough contact to get it adequately hot, or the
     lead had a coating of oxidation that blocked the necessary chemical bonding. In fact,
     that style of cold solder joint is a major cause of factory defects resulting in warranty
     claims. At least it was, back when most components had leads. It doesn’t occur nearly
     as often with leadless, surface-mount parts. Frequently, though, the problem is where
     the solder meets the pad. See Figure 6-9.
             Chapter 6      Working Your Weapons: Using Test Equipment                 99




               Figure 6-7     Proper soldering technique




                   Figure 6-8     Good solder joints


    If the joint looks like a bead sitting on top of the pad, you have not created a
molecular bond and will need to reapply the iron. To get enough heat, the wattage of
the iron has to be appropriate to the size of the joint. Also, you have to apply some
pressure to the tip for effective heat transfer; a very light touch won’t do it. Don’t
press really hard, though, as it won’t improve transfer and could cause damage.
    If the iron’s tip is contaminated, heat transfer will be limited. It should look shiny.
Especially if it has come in contact with plastic, it could have a coating blocking the
heat. Although plastic contamination is most easily removed by scraping the tip when
the iron is cold, tinning and then wiping a hot tip may cut through the coating.
100     How to Diagnose and Fix Everything Electronic




                           Figure 6-9      A cold solder joint. Note how the
                           edge fails to flow into the surrounding pad.



             Contamination can occur on the leads of replacement components, too, especially
        with parts that have sat around for years in your parts drawers. If the leads look dull,
        apply some fine sandpaper or scrape them clean before attempting to solder them.
        They should be shiny for good solder flow.
             Once you have a nice, properly flowed joint, remove the supply of solder and
        then the iron, in that order. If you do have to reflow the joint, add a small amount of
        new solder so you’ll have fresh rosin on the joint to help facilitate bonding.
             Soldering leadless, surface-mount components is tricky, mostly because they
        are so small that it’s hard to keep them in place while applying the iron. Make sure
        the board’s pads are completely flat, with no solder blobs on them, and then put
        the part in place. Hold it down with a small screwdriver placed in the middle of the
        component while you solder one end. Unless you’re anatomically quite unusual, you
        won’t have an extra hand to feed solder to the joint, so put enough solder on the tip
        to make a crude joint. Don’t even worry about molecular bonding. Just tack that side
        down, even if it’s with a bad joint. Then let go of the component and properly solder
        the other side, taking care to make a good joint. Finally, go back to the first side and
        do it right. You might have to wick off your first attempt before trying again. Be extra
        careful not to heat the part too much, or the good side will come unsoldered; those
        tiny parts conduct heat much faster than do larger components with leads. Also, a lot
        of heat can delaminate and destroy the component’s solderable platings. What works
        best is adequate heat applied quickly. Get on and off the part with minimum delay.

              Before trying to solder tiny, surface-mount parts in a device you’re trying to
      Tip     repair, practice on a scrap board. Experience really helps in developing successful
              soldering technique with these minuscule components.
                Chapter 6      Working Your Weapons: Using Test Equipment 101




                       Figure 6-10      Stranded wires twisted together


        After soldering, the board will be left with a coating of rosin on and around the new
   joint. Some techs leave it on, but it’s not a good idea because it can absorb moisture over
   time. Loosen it by gently scraping with the tip of an X-Acto knife or a small screwdriver.
   Wipe up what’s left with a swab wet with contact cleaner.
        To join wires, first twist them together for a solid mechanical connection. If
   the wires are stranded, try to separate the strands a bit and intertwine them when
   twisting the wires together. See Figure 6-10. If you’re using heat-shrink tubing, keep
   it far from the soldering work or it’ll shrink before you get a chance to slip it over the
   joint. And don’t forget to slide the tubing onto one of the wires before entwining and
   soldering them! It really helps you avoid the expletives from having to cut the wires
   and start over.



Desoldering Tools
   You’ll use desoldering tools almost as often as your soldering iron. The two basic types
   are wick and suckers.


   Wick
   For small work, wick is the best choice. It’s easy to control, doesn’t splatter solder
   all over the area and doesn’t run the risk of generating a static charge capable of
   damaging sensitive components. Its only real drawbacks are that it can’t pick up a lot
   of solder at once, it’s a tad expensive and it’s not reusable.
        To wick the solder off a joint, place the wick on the joint and heat it by pressing
   the iron to the other side. Applying a little pressure helps. In this case, don’t put that
   extra drop of solder on the tip first or it’ll flow right into the wick, wasting some of the
   braid’s capacity to soak up the joint’s solder.
        When the wick saturates with solder, pull it and the iron away at the same time. If
   you remove the iron first, the wick will remain soldered to the joint. If desoldering is
   incomplete, clip off the used wick and try again.
102   How to Diagnose and Fix Everything Electronic

           When desoldering components with leads, it can help to form the end of the wick
      into a little curve and press it against the board so that, when heated, it’ll push into
      the hole in which the lead sits and soak up the solder stuck inside. Be extra careful to
      keep the wick up to temperature until you pull it away, or you could lift the copper
      off the board, creating a significant problem.
           Sometimes you are left with a film of solder the wick refuses to soak up. If that
      occurs, try resoldering the joint with a minimum of solder, just enough to wet it down
      a little. Then wick it all up. The fresh rosin of a new joint can help the wick do its job,
      carrying the old solder to the wick with it.


      Suckers
      Solder suckers come in several varieties. The most common are bulbs, bulbs mounted
      on irons, and spring-loaded.
           Bulbs work well when there isn’t a lot of solder to remove; they tend to choke on
      big blobs of it. To use a bulb, squeeze the air out of it, heat up the joint with your iron,
      position the bulb with its nylon tube directly over the molten solder, get the tube into
      the solder and relax your grip on the bulb. Although the end of the bulb is plastic,
      it won’t contaminate your iron’s tip because the plastic used is a high-temperature
      variety that doesn’t melt at normal soldering temperatures.
           After a few uses, the tube may clog with solder. Just push it inside with a screwdriver,
      being careful not to pierce the bulb. If it’s so clogged that you can’t budge the solder,
      pull the tube out and expel the plug from the other end. Eventually, the bulb will fill
      up and you’ll have to remove the tube to empty it anyway.
           When you have a large joint with lots of solder, a spring-loaded sucker is the only
      thing short of a professional vacuum-driven desoldering station that will get most or
      all of the solder in one pass. Cock the spring and then use it like a bulb.
           The fast snap of a spring-loaded sucker can generate a static charge reputed to be
      capable of damaging sensitive parts, especially MOSFET transistors and integrated
      circuit chips of the CMOS variety. To be on the safe side, don’t use one on those kinds
      of parts.


      Rework Stations
      Solder removal gets trickier as parts get smaller. Some of today’s surface-mount
      parts, which have no leads poking through holes in the board, are getting so small
      that traditional soldering and desoldering tools are inadequate for working on them.
      Surface-mounted integrated circuits (IC chips), especially, may have dozens of leads
      so close together that manual soldering is impossible. To cope with the problem,
      advanced repair centers use rework stations. These systems have specialized tips
      made to fit various IC form factors, and they can resolder as well as desolder. Alas,
      rework stations are quite expensive and out of reach of most home-based repairers.
                Chapter 6      Working Your Weapons: Using Test Equipment 103


Power Supply
   When powering a device from your bench power supply, you need to consider several
   factors for successful operation, and to avoid causing damage to the product.


   Connection
   Many battery-powered items also have AC adapter jacks offering a convenient place to
   connect your supply. The usual style of connector is the coaxial plug. See Figure 6-11.
   These plugs come in many sizes, and both inside and outside diameters vary. If
   you can find one in your parts bins that fits, perhaps from an old AC adapter or car
   adapter cord, you’re in luck! Sometimes you can use a plug with a slightly different
   diameter, but don’t force things if it isn’t a good fit. You may find one that seems to fit
   but doesn’t work because the inner diameter is too large, so the jack’s center pin won’t
   contact the inner ring of the plug.
       The polarity of the plug is paramount! Don’t get this backward or you will almost
   certainly do severe damage to the product as soon as you hit the supply’s power
   switch. Coaxial plug polarity is usually printed on the device somewhere near the
   jack, and it will be on the AC adapter as well. Most modern products put positive on
   the tip and negative on the outer sleeve, but not all. Always check for the polarity
   diagram. It should look like one of these:

                                     –        –
                                              +    +        –


        If there’s no adapter jack, or you choose not to use it, you can connect clip leads
   to the battery terminals of most devices. This should work fine with anything using
   standard, off-the-shelf cells like AAs. The convention is to use a red lead for positive
   and a black one for negative, and I strongly suggest you do so to avoid any possible
   polarity confusion, which could be disastrous.
        In a typical case, you open the battery compartment and find a bunch of springs
   and contacts, one set for each cell. Most of them link one cell’s positive terminal to
   the next one’s negative, forming a series string. One spring (the negative terminal)
   and one positive terminal (usually a flat plate or wire) feed the circuitry from each
   end of the string. Which are the two you need?
        Sometimes the positions of the terminals offer mechanical clues. If you see two
   connected directly to the board, or if they’re placed such that they could be, those are
   probably the right ones. Also, if one set of terminals is on a flip-out or removable door,
   that pair is not what you’re looking for. If you find no such hints, use your DMM’s
   continuity feature to determine which terminals are connected to adjacent ones.
   Whatever’s left should be the two magic terminals.
        If you can reach the terminals with probes while the batteries are installed, pull
   the cells out, use your DMM to measure each one and then reinstall them. Add up
   the voltages to get the total series voltage, and then look for it between terminals.
104   How to Diagnose and Fix Everything Electronic




                       Figure 6-11     Coaxial power plug


      Make sure the product is turned off, so it won’t pull the voltage down. Only the two
      correct terminals will show your calculated voltage; any other combination will be
      significantly lower.
          If the device uses a square 9-volt battery, the larger, petal-like terminal of the
      unit’s snap-on connector is positive. Nine-volt gadgets are pretty rare these days,
      but you will encounter this battery style if you work on old transistor radios or tape
      recorders. Some digital answering machines and clock radios still use 9-volt batteries
      for memory backup.
          In products with proprietary lithium-ion batteries, it’s often possible to provide
      power through the device’s terminals, but not always. Some of them, especially
      camcorders and laptops, use “smart” batteries containing their own microprocessors,
      and the device won’t recognize power applied to the terminals without the data those
      micros provide.
          Many smaller items, like digital cameras, may have three terminals. Two are for
      power, of course, and the third one is for a temperature sensor to prevent overheating
      during the charge cycle. Usually, these devices can be powered from a power supply,
      with the third terminal left unconnected.
          To determine which terminal does what, look at the battery. You probably won’t
      find any polarity markings on the product, but they are nearly always printed on
      the battery, and you can place it in the orientation required for insertion and see
      which terminals line up with the ones in the unit. You want the + and – terminals, of
      course. The other one may be marked “C” or have no marking at all.
          If the battery is also unmarked, try measuring its voltage with your DMM. Most
      commonly, the voltage output is from the two outer terminals, with the sensor
      terminal between them. Once you find the right terminals, you’ll also know the
      polarity. This assumes, of course, that the battery has at least a little charge on it;
      there’s no way to read a dead battery, and don’t even think of trying to apply a charge
      from your power supply without knowing the polarity. Lithium-ion batteries are nasty
      when they burst. You can get hurt. Even when you do know the polarity, putting too
      much current through lithiums too fast can make them go boom.
                 Chapter 6      Working Your Weapons: Using Test Equipment 105

    Voltage
    Set the supply’s voltage before connecting it to the device, and try to get it pretty close
    to what the unit expects. Most products list their battery voltages either on the back of
    the unit or on the battery, in the case of proprietary cells. In AA- or AAA-driven units,
    just multiply the number of cells by 1.5. Remember, though, that NiMH (nickel-metal
    hydride) rechargeable cells are only 1.2 volts each. If you run into the rare item that
    is made for use only with rechargeables, multiply by 1.2 instead. Some digital cameras
    and MP3 players fall into that category; they will not function with alkalines or other
    throw-away cells, and the higher voltage may damage them.
         Most hobby-grade power supplies have analog meters, and they can be off by
    quite a bit. To keep things more accurate, use your DMM to set the voltage. Don’t
    worry about millivolts; just stay within half a volt or so and you should be fine. Even
    lithium-ion batteries start out a little above their rated voltage when fully charged,
    with the voltage dropping as the charge is drained. The curve is a lot flatter than with
    other battery technologies, though, which is why it’s a good idea to match the rated
    voltage the best you can.
         Once you’ve set the voltage, turn the supply back off and connect the leads,
    double-checking the polarity. Then, hit the switch and pray. No smoke? Great! You’re
    in business.


    Current
    The current drawn by a device will vary, depending on what the unit is doing.
    Especially with any product employing moving parts like a hard drive platter or laser
    optical head, current demand goes way up during mechanical motion, dropping again
    when movement ceases.
         As long as your supply has sufficient current capacity, it doesn’t matter. If,
    however, the supply has enough for some modes of a device’s operation but not
    others, the results can be unpredictable.
         This issue crops up during service of camcorders and hard drive–based MP3
    players. The drive spins up or the tape loads, and suddenly the device shuts down
    or its micro gets scrambled due to the lowered voltage from an overloaded supply. If
    your supply has a current meter, keep an eye on it to be sure you’re never pulling the
    supply’s maximum current. If the meter does show maximum, the product’s demand
    is probably exceeding what the supply can offer, and the voltage is dropping.



Transistor Tester
    Using a transistor tester requires taking the transistor out of the circuit. Doing so
    ranges from easy, with small-signal transistors, to a hassle, with power transistors
    mounted on heatsinks. Transistors have three leads (see Chapter 7), so you’ll have
    to disconnect at least two of them, though it’s usually easier just to desolder all three
106   How to Diagnose and Fix Everything Electronic

      and pull the part off the board. Big power transistors with metal cases employ the case
      as one terminal, usually the collector. With those, you may find it more convenient
      to leave the part on the board and disconnect the other two terminals, especially if
      they’re connected with wires, rather than directly to circuit board traces.
           There are several basic types of transistors, and testing procedures vary. With some
      transistor testers, you need to know which terminal is which, while others will try out
      all the combinations automatically and recognize when the correct configuration has
      been found.
           Fancy transistor checkers can evaluate a transistor’s characteristics in actual
      operation by using the transistor as part of an oscillator built into the checker. They
      can show you the part’s gain and leakage. For most service work, such sophisticated
      measurement is unnecessary. Usually, you just want to know if the part is open or
      shorted. A transistor can be “leaky,” passing reverse current or allowing flow between
      terminals when it shouldn’t occur, but it doesn’t happen often.
           To test a transistor, connect its three leads as specified in your tester’s instructions,
      and read whatever info it gives you. There are too many types of testers to detail their
      operation here.
           Some DMMs include transistor test functions. If yours has a little round socket
      marked E, B and C, you have a transistor tester!



Capacitance Meter
      Checking capacitors requires disconnecting at least one of their leads, because other
      circuit elements will distort the reading. Be absolutely sure to discharge the capacitor
      before testing it, especially with electrolytic caps, which can store a lot of energy
      capable of trashing your tester.
            Turn the meter on and connect the capacitor to the input terminals. Some meters
      have special terminals into which you can press the cap’s leads. You can use those
      or the normal clip leads, whichever is more convenient. If the capacitor is polarized,
      connect it the right way around, + to + and – to –! See Chapter 7 for polarity marking
      styles.
            If your meter is autoranging, it’ll step through its ranges and read the cap’s value. If
      it isn’t, begin at the most sensitive range, the one that reads pf (picofarads, or trillionths
      of a farad) and work your way up until the “out of range” indicator goes away and you
      get a valid reading. When you’re finished, unhook the capacitor and discharge it. Very
      little energy is put into the component to test it, so you can short across its terminals
      without worry.
            The meter will show you the value of the capacitor in fractions of a farad. Some
      types of capacitors, especially electrolytics, have fairly wide tolerances, or acceptable
      deviations from their printed values. Typically, an electrolytic can be off by 20 percent
      of its stated value even when new. Manufacturers deliberately err on the high side
      to ensure that filtering will be adequate when the caps are used in voltage smoothing
      applications, as many are. If the cap reads a little high, don’t worry about it. If it reads a
      little low, that may be okay too. If it reads more than 20 percent low, suspect a bad cap.
                Chapter 6      Working Your Weapons: Using Test Equipment 107

   And if you can’t get rid of the out-of-range indicator on any scale, the cap is probably
   shorted or very leaky. Open caps will read as extremely low capacitance.
       Only very specialized capacitance meters can read an electrolytic cap’s equivalent
   series resistance (ESR). This parameter goes up as a cap ages, eventually interfering
   with proper operation and rendering the cap useless. A cap can still be bad even
   though it looks fine on a normal cap meter. But if it reads significantly below its
   intended value, or shows a short or an open, it is bad.



Signal Generator
   A signal generator is used to replace a suspected bad or missing signal temporarily so
   you can see what its insertion will do to a circuit’s behavior. Inserting a signal is a very
   handy technique when working on audio circuitry. It can also help you check clock
   oscillator function in digital gear or sub for a missing oscillator in radio equipment.
       For audio testing, it’s best to use a sine wave somewhere in the lower middle of
   the audio spectrum, at around, say, 1 kHz. Using a sine wave prevents the generation
   of harmonics that could damage the amplifier under test, the speakers or your ears.
       For clock oscillator substitution, set the generator to the same frequency as the
   missing oscillator (it should be marked on its crystal), and use a square wave. Set the
   peak-to-peak voltage of the generator just below whatever voltage runs the chip normally
   doing the oscillating. Don’t exceed it, or you could “latch” and damage the chip.
       For radio oscillator substitution, use a sine wave at the frequency of the missing
   oscillator. It’s probably best to feed the signal from the generator through a capacitor
   of around 0.01 µf to avoid loading down the radio’s circuits. Set the peak-to-peak
   oscillator voltage to something less than the power supply feeding the radio’s stage.
   Usually some fraction of a volt is plenty in this kind of experiment.



Frequency Counter
   Frequency counters are used to adjust a device’s oscillators to a precise, accurate
   frequency, or to verify a frequency. Radio and TV receivers, video recorders and
   even some all-digital devices can require carefully set oscillators for proper operation.
   Frequency measurement also may aid in troubleshooting optical disc players.
       A counter works by totaling up how many cycles of an incoming waveform go
   by in a period of time specified by the instrument’s gate period. The gate opens, the
   waves go by, it counts ’em and puts the count on the display. That’s it.
       Ah, if only real life were that simple! Sometimes this works and gives you a correct
   count; sometimes it doesn’t. For one thing, how does the counter know when a cycle
   has occurred? Unlike the trigger on a scope, the counter’s trigger is very simple: it
   looks for zero crossings, or places where the signal goes from positive to negative, and
   counts every two of them as one cycle. For simple waveforms with little or no noise,
   that works great.
108   How to Diagnose and Fix Everything Electronic

           A lot of signals have noise or distortion on them, however, that can confuse the
      zero-crossing detector, resulting in too few or too many counts. If you connect a
      counter to a test point using a scope probe and then switch between 10X and 1X on
      the probe, the count will probably change a little bit, with a lower count in the 10X
      position. Which one is the truth? It’s hard to say for sure. Usually, the count is more
      accurate when the input voltage is lower because noise on the signal is less likely to
      trip the zero-crossing detector. If it gets too low, though, the detector may miss some
      cycles altogether, resulting in an incorrect, low count. If you get counts that seem
      very low for what you were expecting—say, a 10-MHz oscillator reads 7.2 MHz—the
      input voltage is likely too low and the detector is missing some cycles. If the count
      seems too high, the signal may be noisy and also could be too strong, adding false
      cycles to the count.
           Complex, irregular signals like analog audio, video and digital pulse trains cannot
      be counted in any useful way with a frequency counter. What comes in during each
      gate period will vary, so the display won’t settle down. Also, correct tripping of the
      zero-crossing detector is impossible. Use this instrument only for simple, repeating
      waveforms such as those produced by oscillators.
           Back in Chapter 2, we looked at precision and accuracy, and how they affected
      each other. Nowhere does this issue come up more often than with frequency counters.
      Most counters have lots of digits, implying high precision. Accuracy is another matter.
           The count you get depends on how long the gate stays open. That is controlled
      by a frequency reference, which is an internal oscillator controlled by a quartz crystal.
      In a very real sense, the counter is comparing the incoming signal’s frequency to that
      of the instrument’s crystal, so variation in the crystal oscillator’s frequency will skew
      the count. Crystals are used in many oscillator applications requiring low drift, but
      they do wander a bit with temperature and age. Most counters have internal trimmer
      capacitors to fine-tune the crystal’s frequency, but setting them requires either
      another, trusted counter for comparison; an oscillator whose frequency is trusted; or
      some cleverness with a shortwave radio that can receive WWV, the National Bureau
      of Standards atomic clock’s time signal originating from Fort Collins, Colorado. That
      station broadcasts its carrier at a highly precise, accurate frequency, and it is possible
      to compare it audibly to your counter’s oscillator, using the radio as a detector. When
      they zero-beat, or mix without generating a difference tone, your counter is spot on
      frequency. Performed very carefully with a counter that’s been fully warmed up, zero-
      beating against WWV can get you within 1 Hz, which is darned good.
           On many counters, the gate time can be selected with a switch. Longer gate times
      give you more digits to the right of the decimal point, but their accuracy is only as
      good as the counter’s reference oscillator. Don’t take them terribly seriously unless
      you are certain the reference is correct enough to justify them. If your right-most digit
      specifies 1 Hz but the reference is 20 Hz off, what does that digit mean? For audio
      frequencies, you’ll need a fairly long gate time to get enough cycles to count. For radio
      frequencies, a faster gate time is more appropriate.
           Connecting a counter to the circuit under test can be tricky in some cases. Loading
      of the circuit’s source of signal generation is a real problem, pulling it off-frequency
      and affecting the count significantly. Especially when you touch your probe directly
                Chapter 6      Working Your Weapons: Using Test Equipment 109

   to one lead of a crystal in a crystal oscillator, the capacitance of the probe and counter
   can shift the oscillator’s frequency by a surprising amount. To probe such a beast, look
   for a buffer between the oscillator and everything else, and take your measurement
   from its output. Since the circuit itself also may load the oscillator, it’s highly likely
   that a buffer is there someplace. These days, most crystal oscillators are formed with
   IC chips, rather than transistors, with the buffer being on the chip and the frequency
   output coming from a pin not connected to the crystal. Use your scope to find it and
   then connect the counter to that pin.



Analog Meter
   Using an analog VOM or FET-VOM requires interpreting the position of a meter needle,
   rather than just reading some numbers off a display. Why would you bother with this?
   That meter needle can tell you some things a numerical display can’t. Specifically,
   how it moves may give you insight into a component’s or circuit’s condition.
        Taking most kinds of measurements with a VOM is pretty much like taking them
   with a DMM. VOMs are not autoranging, so you have to match the selector knob’s
   scale to the markings on the meter movement. Also, you need to zero the ohms scale
   with the front-panel trimmer knob every time you change resistance ranges. Select
   the desired range, touch the leads together and turn the trimmer until the meter reads
   0 ohms.
        Unlike FET-VOMs and VTVMs, VOMs have no amplification, so they load the
   circuit under test much more when reading voltage than do other instruments.
   This is of no consequence when measuring the output of a power supply, but it can
   significantly interfere with some small-signal circuits.
        VOMs can pull a few tricks not possible with their digital replacements. Slowly
   changing voltages will sway the meter needle in a visually obvious way, instead of
   just flashing some numbers. A little noise won’t affect the reading, either, thanks
   to the needle’s inertia. A lot of hum on a DC signal can vibrate the needle in a very
   identifiable manner. Some old techs could read a meter needle almost as if it were
   a scope!
        If you don’t have a capacitance meter, you can gauge the condition of electrolytic
   capacitors with the meter’s ohms scale. This works pretty well for caps of about 10 µf
   or more; it doesn’t work at all for anything under 1 µf or so. Set the meter to its highest
   range and touch the test leads together. When the needle swings over, use the trimmer
   on the front panel to adjust it to read 0 ohms. (If the needle won’t go that far, the meter
   needs a new battery!)
        Connect the leads to the discharged cap, + to + and – to –, and watch what
   happens. The meter should swing way over toward 0 ohms and then gradually fall
   back toward infinity. The greater the capacitance, the harder the needle will swing,
   and the longer it’ll take before it finally comes to rest. If the needle doesn’t fall all
   the way back, the cap is leaky. If it doesn’t swing toward zero, it’s open or of low
   capacitance.
110   How to Diagnose and Fix Everything Electronic

          The meter applies voltage from its battery to the capacitor, so be sure the cap’s
      voltage rating is higher than the battery voltage or you could damage the cap. Some
      meters use 9-volt batteries for their higher resistance ranges. If yours does, it’s wise
      not to try this test on caps rated lower than that. If your meter uses only an AA cell,
      there’s nothing to worry about.
          The ohms scale can also be used to check diodes and some transistors, just as with
      a DMM.



Contact Cleaner Spray
      Cleaner spray is handy stuff, especially for use with older, analog equipment. Volume
      controls, switches and sockets can all benefit from having dirt and oxidation cleaned
      away. When controls exhibit that characteristic scratchy sound, it’s time to get out
      the spray.
          For spray to be effective, you have to be able to get it onto the active surface of the
      control or switch. Sometimes that’s not easy! If you look at the back of a potentiometer,
      or variable resistor, you may find a notch or slot giving you access to the inside, where
      the spray needs to be. Always use the plastic tube included with the spray can! Trying
      to spray the stuff in with the bare nozzle will result in a gooey mess all over the inside
      of your gear. Once you get some spray into the control, rotate it back and forth a
      dozen times. That’ll usually cure the scratchies.
          Switches can be a bit tougher. Some simply have no access holes anywhere. If you
      can’t find one, you’ll have to spray from the front, into the switch’s hole. Never do this
      where the spray may come into contact with plastic; most sprays will permanently
      mar plastic surfaces.
          Trimmer capacitors should not be sprayed. They have plastic parts easily damaged
      by the spray and may lose function after contact with it.
          Do your best not to splatter spray onto other components. Wipe it off if you do.
      Also, it could shatter a hot lamp, so don’t use it near projector bulbs. And, of course,
      avoid breathing it in or getting it in your eyes. Spray has a nasty habit of reflecting
      back out of the part you’re blasting, right into your face. Keep your kisser at least
      12 inches away. Farther is better.
          To avoid making a mess or getting soaked, try controlling the spray by pressing
      gently on the nozzle until only a gentle mist emerges. Some cans have adjustable
      spray rates, but many don’t. Some brands are more controllable than others, too.
      Experiment with this before you attack expensive gear.



Component Cooler Spray
      Cooler spray can be incredibly useful for finding thermal intermittents. If a gadget
      works until it warms up, or it works only after it warms up, normal troubleshooting
      methods can be hard to implement, especially in the second case. How are you going
      to scope for trouble in something that’s working?
            Chapter 6      Working Your Weapons: Using Test Equipment 111

     Before hitting parts willy-nilly with cooler, decide what might be causing the
trouble. The most likely candidates for cooler are power supply components like
transistors and voltage regulators, output transistors and other parts with significant
temperature rises during normal operation.
     As with cleaner, use the spray tube, and try controlling the spray rate. Also, the
same caveats regarding breathing it in and getting hit in the face hold here. This stuff
is seriously cold and can damage skin and eyes. A small amount hitting your hands
won’t do you any harm, but I shudder to think of a single drop’s splattering on your
cornea.
     If spraying a suspected component reverses the operational state of your device
(it starts or stops working), you’ve most likely found the trouble.
     After spraying, moisture will condense on the cold component. Be sure to kill the
power and wipe it off.
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Chapter                    7
   What Little Gizmos Are Made of:
   Components

   A    lthough there are hundreds of types of electronic components and thousands of
        subtypes (transistors with different characteristics, for example), a small set of
   parts constitutes the core of most electronic products. Let’s look at the most common
   components and how to recognize and test them. In this chapter, we’ll cover out-of-
   circuit tests, the kind you perform after removing the part from the board. We’ll get to
   in-circuit testing in Chapter 11, when we explore signal tracing and diagnosis.



Capacitors
   Capacitors consist of two plates separated by an insulator. A charge builds up on the
   plates when voltage is applied, which can then be discharged back into the circuit.
   Capacitors come in many types, including ceramic, electrolytic, tantalum, polystyrene
   (plastic) and trimmer (variable). See Figure 7-1.

   Symbols


   Markings
   Most capacitors are marked in straightforward manner, with numbers followed by
   µf (microfarads, or millionths of a farad) or pf (picofarads, or trillionths of a farad).
   Leading zeros to the left of the decimal point are not shown, so we won’t use them
   here either. Some European gear has caps marked in nf (nanofarads, or billionths of
   a farad). Thus, a cap marked 1 nf = .001 µf. Some capacitors are marked with three
   numbers, with no indication of µf, nf or pf. With these, the last number is a multiplier,
   indicating how many zeros you need to tack on, starting from picofarads. For instance,
   a cap marked 101 is 100 pf, because there is one zero after the two numbers indicating


                                                                                       113
114   How to Diagnose and Fix Everything Electronic




                     Figure 7-1  Ceramic disc, plastic, electrolytic,
                     surface-mount electrolytic, tantalum, and trimmer
                     capacitors



      the value. A marking of 102, then, is 1000 pf, or .001 µf. And 103 is 10,000 pf or .01 µf,
      and so on. Here’s a handy list:

       •	 XX0 = less than 100 pf. The two Xs are the value in pf. Sometimes there’s no zero
          on the end. A tiny cap marked 27 is 27 pf. One marked 270 is also 27 pf.
       •	 XX1 = value X 10 pf
       •	 XX2 = .00XX µf
       •	 XX3 = .0XX µf
       •	 XX4 = .XX µf
       •	 XX5 = X.X µf
       •	 XX6 = XX µf

      Any value greater than these will be marked directly in µf, as in “2000 µf.”
           On ceramic disc capacitors, you may also see a marking like N750. This specifies
      the temperature coefficient, or how much the capacitance drifts with temperature and
      in which direction. Keep an eye out for NP0, which means no drift in either direction.
      NP0 caps are used in time constants and tuned circuits so they won’t change frequency
      as the unit warms up. Should you ever need to replace an NP0 cap, be sure to use the
      same type.
           Polarized capacitors are marked for their polarity. With can-style electrolytics, the
      marking is a long arrow or black line, and it indicates the negative lead. Some very old
      electrolytics may show a + sign instead, indicating the positive lead, but they haven’t
      been made that way for many years. Look for them in antique radios and such.
           Tantalum electrolytics, which look like little dipped candy drops with wires, denote
      the positive lead with a + sign, or sometimes a red or silver dot.
             Chapter 7      What Little Gizmos Are Made of: Components 115

    Surface-mount electrolytics of the can type are marked with a black line or
semicircle, usually on the top, indicating the negative lead. Flat plastic caps have one
end painted silver or white, and this denotes the positive terminal; don’t confuse it
with the negative-indicating line on can-style caps. Some small surface-mount caps
of well under 1 µf have no markings at all, making it impossible to determine their
values without a capacitance meter. They are usually tan and are neither polarized
nor electrolytic.
    If you find a can-type capacitor with no polarity marking, look for NP, which
indicates a non-polarized electrolytic cap. These are uncommon, but you sometimes
run across them in audio gear. You must replace NP caps with the same type.


Uses
Different styles of capacitors cover various ranges and are used for different purposes.
Here are the common ones:

 •	 Ceramic These cover the very small values, from a few pf up to around .1 µf,
    and are used in resonant radio circuits and bypass applications.
 •	 Plastic These start at around .001 µf (1000 pf) and may go as high as .47 µf or
    so. They are used for bypass and coupling, and are sometimes found in time
    constants because of their excellent stability over a wide range of temperatures.
 •	 Electrolytic These start at around .47 µf and cover the highest ranges, on up to
    tens of thousands of µf. They are used for coupling and filtering.
 •	 Tantalum These range from .1 µf to around 47 µf and are a special type of
    electrolytic capacitor with lower impedance at high frequencies. They are used in
    filtering and bypass applications when high frequencies are present.
 •	 Trimmer These range from the low pfs to around 200 pf and are used as frequency
    adjustments for tuned (resonant) circuits and oscillators.


What Kills Them
Different styles of capacitors fail for different reasons, depending on how they’re used
and to what conditions they’re subjected. Generally, application of a voltage above the
cap’s ratings can punch holes in the dielectric (insulating) layer, heat can dry or crack
them, and some wear out with age.

Ceramic and Plastic These very rarely fail. In all my years of tech work, I’ve found
two bad ceramics and one bad plastic cap! It just doesn’t happen. If these caps appear
unharmed, they are almost certainly okay.

Electrolytic These are the most failure-prone components of all. Part of their
charge-storing layer is liquid, and it can dry out, short out, swell and even burst the
capacitor’s seals and leak out. Heat, voltage and age all contribute to their demise. The
constant charging and discharging as they smooth ripple currents in normal operation
116   How to Diagnose and Fix Everything Electronic

      gradually wears the caps out. Today’s predominance of switching power supplies,
      with their fast pulse action, has accelerated capacitor failure. And application of even
      a little reverse-polarity current will wear out ’lytics in a hurry. A leaky rectifier will
      permit some AC to hit the filter caps connected to it, resulting in ruined caps. If you
      replace them without changing the rectifier, the new parts will fail very quickly.
           Failure modes include shorts, opens, loss of capacitance from age or drying out,
      electrical leakage (essentially a partial short), and increased internal resistance,
      or equivalent series resistance (ESR). If you see a bulge in the top of the cap, or
      anywhere on it, for that matter, it is bad and must be replaced. Don’t even bother to
      check it; just put in a new one. Look at the bowed top of the capacitor in Figure 7-2.
      Keep in mind that a cap can also exhibit high ESR or decreased capacitance with no
      physical signs.
           Many surface-mount electrolytics made in the 1990s leaked, thanks to a defective
      electrolyte formula. If the solder pads look yellow or you see any goo around the cap,
      the part has leaked and must be replaced. Sometimes the yellow pads are the only
      clue.

      Tantalum These use a solid electrolyte in the dielectric layer that is quite thin.
      Consequently, they are prone to shorts from even momentary voltage spikes exceeding
      their voltage ratings and punching holes in the layer. And they are even less tolerant of
      reverse current than are standard electrolytics.

      Trimmer Trimcaps use a plastic or ceramic insulating layer that is very reliable. They
      rely on a mechanical connection, though, between the rotating element and the base,
      making them prone to failure from corrosion over a period of many years. Sometimes
      rotating the adjustment through its range a few times can clear it up, but doing so




                     Figure 7-2    Bulging electrolytic capacitor with
                     greatly reduced capacitance
                Chapter 7       What Little Gizmos Are Made of: Components 117

   causes loss of the initial setting, requiring readjustment afterward. Never spray the
   plastic variety with cleaner spray, as it may damage the dielectric layer and destroy the
   component.


   Out-of-Circuit Testing
   To test a capacitor after removal from the circuit, use your cap meter. If you don’t
   have one, you can use your DMM’s ohms scale to check for shorts. Small-value caps
   will appear open whether they are or not; they charge up too quickly for you to see
   the voltage rising.
        For electrolytics, do a quick test with an analog VOM, watching for the initial needle
   swing and slow release back toward infinity. Make sure the cap is discharged before
   you try to test it. With a polarized cap, connect the test leads + to + and – to –. Most
   polarized caps will survive a reversed test, but tantalums may not; even momentary
   reversed voltage can short them out.



Crystals and Resonators
   Quartz crystals and ceramic resonators are made from slices of quartz or slabs of
   ceramic material, with electrodes plated on the sides. They exploit the piezoelectric
   effect, in which some materials move when subjected to a voltage and also generate a
   voltage when mechanically flexed. Crystals are always found encased in metal, while
   ceramic resonators are usually in yellow or orange plastic. Crystals have two leads,
   and ceramics may have two or three. See Figure 7-3.




                     Figure 7-3     Quartz crystals and ceramic resonators
118   How to Diagnose and Fix Everything Electronic

      Symbols Crystals and resonators use the same symbol unless
      the resonator features internal capacitors and has three leads.


      Markings
      Crystals and resonators are marked with their frequencies. For crystals, assume the
      number means MHz. On resonators, assume hundreds of kHz, though some may be
      in MHz as well. Crystals can sport other numbers indicating the type of cut used,
      which is quite a complicated topic. It’s not a concern in most service work, though.
      Either the crystal oscillates or it doesn’t.


      Uses
      Quartz crystals are used as frequency-determining elements in oscillators, and
      sometimes as tuned filters in radio applications. Ceramic resonators are used the
      same way, but in applications requiring less stability and accuracy, and usually at
      lower frequencies. You are more likely to find a quartz crystal in the clock oscillator
      running a digital device like a laptop, DVD player or MP3 player, with a ceramic
      resonator lurking in a remote control or some radio circuit stages.


      What Kills Them
      Crystals and resonators are mechanical. They actually move on a microscopic level,
      vibrating at their resonant frequency. They are also made of crystalline material, so
      they’re somewhat brittle. Heat and vibration can crack them, as can a drop to the
      floor. Quartz crystals, especially, can develop tiny internal fractures and just quit on
      their own, with no apparent cause.
           Some flaws don’t stop them outright; they become finicky and unpredictable.
      Touching their terminals may cause them to stop or start oscillating. Also, crystals
      drift in frequency as they age, sometimes drifting past the point at which the circuit
      will operate properly.


      Out-of-Circuit Testing
      Without a crystal checker, which is simply an oscillator with an indicator light, there’s
      no way to tell whether a crystal works without scoping its signal in an operating
      circuit. Even a crystal checker may lie to you, indicating a good crystal that still won’t
      start in the circuit for which it’s intended.


Crystal Clock Oscillators
      Crystal clock oscillators are complete clocking circuits in a small metal box with four
      or six pins. Figure 7-4 shows a really small one. (You can see a much larger unit in
      Figure 10-8, in Chapter 10.) On four-pin parts, two pins are for power and ground,
             Chapter 7       What Little Gizmos Are Made of: Components 119




                   Figure 7-4     12-MHz crystal clock oscillator


a third for output, and the fourth, called output enable, to activate or inhibit the output.
Six-pin versions sport two complementary outputs (one is high while the other is low),
output enable, power, ground and one unconnected pin.

Symbols

                      Vcc   OUT           Vcc    OUT     OUT



                       4      3            6       5       4
                       1      2            1       2       3



                      OE                  OE      NC



Markings
The frequency will be marked on the case. You may also see a manufacturer’s part
number.


Uses
More and more, crystal clock oscillators are replacing separate crystals, especially in
products using multiple frequencies for various tasks. Because the oscillator is in the
can, no extra circuitry is required, so cost and required space are reduced.
120   How to Diagnose and Fix Everything Electronic

      What Kills Them
      These oscillators contain quartz crystals, so they’re subject to the same mechanical
      issues inherent in crystals. Because the cans also include a complete circuit, they’re
      vulnerable to heat and overvoltage failures as well. For the most part, though, crystal
      can oscillators are very reliable.


      Out-of-Circuit Testing
      Applying power and ground to the appropriate pins should produce an output
      waveform at the frequency shown on the can. The amplitude should be fairly close
      to the applied DC voltage. Unless no power is getting to the oscillator, it’s just as
      easy to test one in-circuit as out. If there’s power, the waveform should be there. The
      output enable pin must not be low, or no output will appear. It’s fine for it to be left
      unconnected, and in many circuits it is, since the oscillator needs to run all the time
      anyway.


Diodes
      Diodes are one-way valves. Current can flow from their cathodes (–) to their anodes
      (+) but not the other way. They are made from silicon slabs “doped” with materials
      that cause the one-way current flow, with two dissimilar slabs touching at a junction
      point. Large diodes used in power supplies are called rectifiers, but they do the same
      thing. Two of them in one package, sharing one common terminal (for a total of
      three leads), are called a double diode, or double rectifier. Four of them arranged in a
      diamond-like configuration are called a bridge rectifier, whether they are separate parts
      or integrated into one package. See Figure 7-5.




                    Figure 7-5     Diode, rectifier, and bridge rectifiers
             Chapter 7      What Little Gizmos Are Made of: Components 121


Symbols

                                                                   LED




Markings
Single diodes and rectifiers have a band at one end indicating the cathode. They are
marked by part number, which does not indicate their operating parameters like
maximum forward and reverse operating voltages and maximum current. To get
those, you must look up the part number in a book or online.
    Bridge rectifiers may be marked for peak forward voltage and current, but they
often have part number markings or none at all. Sometimes you’ll see “~” at the AC
inputs and “+” and “–” at the DC outputs.
    You may see a marking like 200 PIV, for peak inverse voltage. This is the maximum
voltage the diode can withstand in the reverse (nonconducting) direction before
breaking down and allowing the voltage to pass. Exceeding the PIV rating usually
destroys the diode.


Uses
Diodes and rectifiers are common parts in pretty much every electronic product.
Small-signal diodes rectify signals for detection of the information they carry, as in
a radio, and direct control voltages that turn various parts of the circuitry on and off.
They may also be used as biasing elements, providing a specific current to the inputs
of transistors and other amplifying elements, keeping them slightly turned on so that
they can conduct over the required portion of the input signal’s waveform. Light-
emitting diodes, or LEDs, are used as indicators on control panels, and bright white
LEDs backlight newer LCD screens in many products, from laptops, netbooks, and
tablet computers to large TVs.
     Rectifiers convert incoming AC power to DC. They convert the pulses in a switching
power supply’s transformer back to DC as well. Bridge rectifiers are commonly used
to change AC line current to DC by directing opposite sides of the AC waveform to the
appropriate + and – output terminals.


What Kills Them
Diodes and rectifiers can fail from voltage exceeding their limits, but the most common
cause is too much current and the heat it produces. Sometimes they just fail with age,
too. Failure modes include opens, shorts and leakage.
122   How to Diagnose and Fix Everything Electronic

      Out-of-Circuit Testing
      Most DMMs include a diode test function that shows the voltage drop across the part,
      indicating whether it is passing current. Be sure to test the diode in both directions. It
      should show a drop of around 0.6 volts in one direction and appear open in the other.
      An open reading will show the applied test voltage, the same as when the meter’s
      leads are unconnected to anything. Even a small voltage drop in the reverse direction
      suggests a leaky diode that should be replaced.
           With an analog VOM, you can use the resistance ranges to get a good idea of
      a diode or rectifier’s condition, but small leakage currents are hard to detect. With
      either type of meter, you’re most likely to see a total failure, either open or shorted,
      if the part is bad. Leaky diodes are rare, but not so rare that you shouldn’t keep the
      possibility in mind.



Fuses
      Fuses protect circuitry and prevent fire hazards by stopping the current when it
      exceeds the fuses’ ratings. Though simple in concept, fuses have a surprising number
      of parameters, including current required to blow, maximum safe voltage, maximum
      safe current to block and speed of operation.
           The primary parameter is the current required to melt the fuse’s internal wire and
      blow it. If you’re not sure of anything else, be sure to get that right when replacing
      a fuse.
           The speed at which the fuse acts is also important in some devices. Time-delay or
      slow-blow fuses are used for applications like motors, which may require momentary
      high start-up current. Ultra-fast-acting fuses are used with especially sensitive circuitry
      that must be protected from even transient overcurrent conditions. Most consumer
      electronic gear uses standard fuses, which are considered fast-acting but not ultra-fast.
           Fuses come in many shapes and sizes, from the ubiquitous glass cylinders with
      metal end caps to tiny, rectangular, surface-mount parts hardly recognizable as what
      they are. You’ll find fuses in holders and also soldered directly to circuit boards. Be on
      the lookout for glass fuses with internal construction including a spring and a little
      coil; those are the slow-blow variety. See Figure 7-6.

      Symbol


      Markings
      At least four marking systems are used on fuses. The primary marking is the melting
      current, shown by a number followed by an A. You may also see 3AG or AGC, both of
      which indicate standard-speed glass fuses. You must look up other markings to get the
      speed rating. Many online catalogs offer this information.
              Chapter 7      What Little Gizmos Are Made of: Components 123




                  Figure 7-6     Fuses



Uses
Anything that plugs into the wall will have a fuse on the hot side of the AC line, usually
right as it enters the device, before it even gets to the power switch. Battery-operated
gadgets often have fuses too, between the battery’s positive terminal and the rest of
the unit. Some products have multiple internal fuses protecting various parts of the
circuitry. I’ve seen as many as eight of them in one camcorder!
    Some audio amplifiers and receivers use fuses in line with the speakers to protect
the amp, should a speaker’s voice coil overheat and short out. Such a fuse can also
protect the speaker if the amp develops a shorted output transistor and sends the
power supply’s entire current capacity straight to the speaker.


What Kills Them
Most fuse failures are caused by doing their job. A short in the circuitry pulls too
much current through the fuse, so it blows. Now and then, you may run across a fuse
that has fatigued with age and use, finally failing. If it’s a glass fuse, take a look at the
inside. When the two wires are almost touching and there’s no discoloration on the
glass, the fuse blew gently, and there’s a possibility that the circuitry isn’t shorted.
That happens sometimes with speaker fuses when the amplifier is played at high
volume for extended periods. The fuse’s wire gets just warm enough to fracture, but
there’s no malfunction in the circuitry. If you see a wide gap between the wires and
spattering on the inside of the glass, the fuse blew hard, indicating a lot of current and
certain circuit failure.
124   How to Diagnose and Fix Everything Electronic

          Very rarely, fuses can develop resistance, continuing to pass current but interfering
      with the full flow. I’ve seen it a couple of times, and my first such case drove me bonkers
      trying to figure out why the power supply voltage was low and erratic. Figuring a fuse
      was either good or bad, I never considered it as a possible culprit until I’d wasted hours
      looking at everything else. Incredulous, I pulled it and discovered that it had become a
      10-ohm resistor! I haven’t trusted the little buggers since.


      Out-of-Circuit Testing
      Check fuses with your DMM’s ohms scale. It should read 0 ohms. If you see an ohm
      or 2, try touching the meter’s leads together; you may get the same reading, thanks
      to the resistance of the leads themselves. If not, and the resistance is definitely in the
      fuse, replace it. A blown fuse will read completely open, of course.



Inductors and Transformers
      Inductors, or coils, generate a magnetic field when current passes through them.
      When the current through the inductor stops or changes direction, the field collapses
      and creates a current in the wire, opposing the changes. The effect is to store some of
      the energy and put it back into the circuit.
          Coils may be wound on nonferrous cores having no effect on the magnetic field, or
      they may be wound on iron cores that play a significant role in the inductor’s behavior.
      Two inductors wound on the same iron core can be used to convert one voltage and
      current to another by creating a magnetic field in one coil and using it to generate a
      current in the other coil, which may have a different number of turns of wire, thus
      creating a different voltage. This arrangement is called a transformer. See Figure 7-7.




                        Figure 7-7     Inductors and transformers
             Chapter 7      What Little Gizmos Are Made of: Components 125

Symbols




Markings
Many coils are not marked. Some small ones are encapsulated in plastic or ceramic
and marked in µh (microhenries, or millionths of a henry) or mh (millihenries, or
thousands of a henry).


Uses
Inductors and transformers are used to convert voltages; to couple signals from one
stage to another, especially in radio equipment; and to filter, or smooth, voltage
variations. They are also essential parts of many resonant circuits, working together
with capacitors to establish time constants.
     Their inherent opposition to rapid voltage change makes coils useful for isolating
radio-frequency signals within circuit stages while allowing DC to pass. When used in
this manner, coils are called chokes because they choke off the signal. Look for them
where DC power feeds from the power supply into RF stages.


What Kills Them
In low-power circuits, coils are highly reliable. Failure is pretty much always due to
abuse by other components. It’s very rare for small-signal inductors to fail, since little
current passes through them. In circuits where significant supply current is available,
a shorted semiconductor can pull enough through the inductor feeding it to blow the
coil. If you find an open inductor, assume something shorted and killed it.
     The windings in power transformers sometimes open when too much current
overheats them and melts the wire. Often, the primary (AC line) side will burn
out even though the excessive current draw is on the other side of the transformer,
somewhere in the circuitry. They may also short to the iron core. The insulation
of the windings in high-voltage transformers can break down and arc over to other
windings, create a short between windings, or arcing and shorts to the core.


Out-of-Circuit Testing
Use your DMM to test for continuity from one end of a coil or section of a transformer
to its other end. You can also use the ohms scale to check for shorts from windings to
the core and between unconnected windings.
     It’s very hard to tell whether windings are shorted to each other in the same coil.
Using the ohms scale, the difference can be so slight that it’s undetectable. If you have
an inductance meter and know the correct inductance, that will give you some idea.
With unmarked coils and just about all transformers, though, you won’t know what
126   How to Diagnose and Fix Everything Electronic

      the inductance should be without access to an identical part for comparison. In some
      circuits, such as LCD backlighting inverters, there can be two identical transformers,
      so you may be in luck.



Integrated Circuits
      ICs, or chips, come in thousands of flavors. In today’s advanced products, ICs do most
      of the work, with transistors and other components supporting their operation. Digital
      chips are at the hearts of computers, DVD players, digital cameras, MP3 players, you
      name it. In many devices, they work side-by-side with analog ICs handling radio,
      audio and video signals. See Figure 7-8.
           ICs integrate anywhere from dozens to millions of transistors on a small square of
      silicon, with microscopic structures printed using photographic techniques. A failure
      of even a single transistor will render the chip defective. It’s pretty amazing that they
      ever work at all! Obviously, there’s no way to test the individual structures; all you
      can do is verify whether the chip is properly performing its intended function.
           There are some off-the-shelf chips used in many products, but custom ICs specific
      to a model or product category are quickly coming to dominate our gadgets’ innards.
      Each chip can include more product-specific functions, so it takes fewer of them to
      make a device. The fewer parts and interconnections, the more reliable an item is
      likely to be. And that gadget can be smaller and cheaper to build.
           Large-scale-integrated chips, called LSIs, can have up to a few hundred pins spaced
      so closely that you can’t even put a probe on one without shorting it to its neighbor.
      Without exceedingly expensive, specialized equipment, it’s very difficult to unsolder or




                LSI
                                                                                    SSI




                        Figure 7-8     Small-scale and large-scale ICs in a
                        DVD player
                 Chapter 7           What Little Gizmos Are Made of: Components 127

resolder these parts. There is a trick for touching up suspected bad joints on some of
them, but it takes practice and doesn’t always work. We’ll look at it in Chapter 12.
     Luckily, ICs are very reliable. Most of them handle small signals, so they don’t
dissipate a lot of power and get hot enough to fail. There are some exceptions to that,
however—notably CPUs and video graphics chips in computers. Some graphics cards
have fans over the chips, as do CPUs. You know the thing gets mighty warm when
it needs its own fan! These parts get so hot because they have millions of transistors
switching from millions to a few billion times per second. Those microscopic heat
generators add up to a serious temperature rise.
     Other hot-running chips include audio power output modules in stereo receivers,
some types of voltage regulators, motor controllers, convergence chips in big-screen
CRT TVs and anything else that pumps real power. These parts are usually mounted
on heatsinks, and they fail as often as power transistors.

Symbols Chips are denoted on schematics by the number of pins and their general
shape. Some simple chips, like logic gates and op amps, may include a schematic of
their general internal functions, but not the actual layout of transistors on the chip.

     14     13       12        11    10       9         8   14        13        12        11         10        9         8




     1      2        3         4     5        6         7   1         2         3          4         5         6         7
                                                                 33
                                                                      32
                                                                           31
                                                                                30
                                                                                     29
                                                                                          28
                                                                                               27
                                                                                                    26
                                                                                                         25
                                                                                                              24
                                                                                                                   23



                                                            34                                                      22
                                                            35                                                      21
      10
           9
                8
                     7
                          6
                               5
                                    4
                                         3
                                              2
                                                   1




                                                            36                                                      20
                                                            37                                                      19
                                                            38                                                      18
                                                            39                                                      17
                                                            40                                                      16
                                                            41                                                      15
                                                            42                                                      14
                                                   20
           12
                13


                          15
                               16
                                    17
                                         18
                     14




                                              19
      11




                                                            43                                                      13
                                                            44                                                      12
                                                                                                              10
                                                                                                                   11
                                                                 1
                                                                      2
                                                                           3
                                                                                4
                                                                                     5
                                                                                          6
                                                                                               7
                                                                                                    8
                                                                                                         9
128   How to Diagnose and Fix Everything Electronic

      Markings
      The parts are marked with part numbers that can mean many things. Small-scale,
      industry-standard chips are produced in families sharing some part number
      commonality. For instance, older CMOS logic gate chips are called “4000 series”
      and have part numbers like 4011B and 4518. They all begin with 4.
          Large-scale integrated circuits often have proprietary part numbers, and they
      certainly will if they’re custom parts made for the specific type of product.
          The pins on an IC are numbered going counterclockwise around the chip in a ring.
      Pin 1 will have a dimple or spot next to it on the chip’s plastic casing, and it will be in
      a corner.


      Uses
      ICs are used for just about everything: audio amps, data processors, logic gates,
      oscillators, signal processing, and any other function you can think of.


      What Kills Them
      ICs are highly reliable, but heat is a major danger, especially when it’s internally
      generated. Voltage spikes can destroy some chip families, but more modern varieties
      are fairly voltage-tolerant. Still, a static charge may present a voltage too high for any
      chip to withstand. A short in another area of the circuit that pulls too much current
      from a chip’s output can blow it. If the power supply feeding the chip fails but signals
      are still fed to it, the chip can latch, causing a permanent internal short. Finally, some
      ICs, especially LSIs, have internal features so small that atomic forces may gradually
      eat through the microscopic wires and connections, creating holes that wreck the
      chip. Essentially, the part dies of old age. This problem has been researched by chip
      makers for many years, and today’s ICs hold up rather well. Now and then, a chip still
      dies without apparent provocation.


      Out-of-Circuit Testing
      There’s no way to put a meter on a chip to see if it’s good. In fact, doing so may
      damage the chip if the meter’s test voltage happens to be high enough and touches
      the wrong pins. Simple logic gates can be plugged into a test board and hooked up to
      test their functions, but the exercise is more trouble than it’s worth. For the most part,
      chips must be tested in-circuit by observing their actions with your oscilloscope.



Op Amps
      The op amp, or operational amplifier, is a common type of chip. It’s an analog,
      general-purpose amplifier configurable to do many different jobs, such as buffering,
             Chapter 7       What Little Gizmos Are Made of: Components 129




            Figure 7-9     Quad op amp



oscillating and signal filtering, by connecting resistors and capacitors to its inputs and
outputs. The number of op amp circuit variations is staggering!
    One chip may contain several op amps. Dual and quad op amps are used in many
products. See Figure 7-9.

Symbol

                                         Vs+

                                     +         Vout

                                     –

                                         Vs–




Markings
The chips are marked with part numbers, and there are some, like LM358, you’ll see
often.


Uses
You’ll find op amps in audio circuits, radio circuits, motor controllers and power supplies.
Any place requiring an amplifier is a prime candidate for an op amp. Most op amps are
for small-signal applications, but some power op amps are capable of driving significant
loads. Expect those to be heatsinked.
130   How to Diagnose and Fix Everything Electronic

      What Kills Them
      Heat and overcurrent are the primary culprits.


      Out-of-Circuit Testing
      As with logic gates, op amps have to be in some kind of circuit for you to test them.



Resistors
      Resistors offer opposition to current, dissipating the opposed power as heat. Limiting
      current is a vital function in any circuit, so every electronic product has resistors.
      They come in many shapes and sizes, but those with leads are easily identifiable by
      their color bands indicating the resistance value in ohms. Some large resistors are
      marked numerically, as are some of the tiny surface-mount parts. See Figure 7-10.
           The basic type of resistor, found in virtually everything, is the carbon composition
      resistor. Made from a carbon compound, these resistors run the full range of values,
      from less than 1 Ω (ohm) to 10 MΩ (megohms, or millions of ohms). Most you’ll see
      will be over 10 Ω and under 1 MΩ. The tolerance for standard carbon comp resistors
      is ± 5 percent. That is, the measured value should be no more than 5 percent high or
      low of the stated resistance.
           Some applications require the use of wire-wound resistors. These look a lot like
      carbon comp parts, but you can see the coil of wire under the paint on the body.
      Wire-wound resistors can dissipate more heat than can carbon comps. They also can
      be manufactured to very tight tolerances, but they have some inductance, since they




                                  Figure 7-10     Resistors
               Chapter 7     What Little Gizmos Are Made of: Components 131

are coils, and are ill suited to high-frequency circuits where the inductance might
matter. The two types should not be interchanged.
     In addition to the resistance value, the power dissipating capability of a resistor,
measured in watts, is an important factor. Since power is dissipated as heat, knowing
how much heat the part can take before disintegrating is vital. Standard carbon comps
are rated at 1/4 watt, with some slightly larger ones able to dissipate 1/2 watt. Very
tiny ones with leads are rated at 1/8 watt, while surface-mount versions typically vary
from 1/8 watt down to 1/32 watt.
     Other resistor formulations include carbon film, metal film and metal oxide. Carbon
film types introduce a bit less noise into the circuit and are used in areas where that’s a
significant issue. Metal film and metal oxide parts have tighter tolerances, in the range
of 1 or 2 percent of stated value. Some circuits require that for proper operation.

Symbol


Markings
The use of color-coding dates back to the vacuum tube days, when resistors got so
hot that printed numbers would evaporate. To read the color code, you must first
determine which end of the resistor is the start and which is the far end. Look for a
gold or silver band; that’s the tolerance marking, indicating the far end, and there’s
usually a little extra space between that band and the others. The first digit will be at
the end farthest from the tolerance band.
    Each number is represented by a color. The scheme is as follows:

       Black                 0                 Green                5
       Brown                 1                 Blue                 6
       Red                   2                 Violet               7
       Orange                3                 Gray                 8
       Yellow                4                 White                9

    The tolerance bands at the far end are as follows:

             Brown                     ± 1 percent
             Red                       ± 2 percent
             Gold                      ± 5 percent
             Silver                    ± 10 percent

    To determine a resistor’s value, read the first two bands as numbers. The third
band is a multiplier, indicating how many zeros you need to tack on to the numerical
value. So, for instance, red-red-brown would be 2, 2, and one zero, or 220 ohms.
Yellow-violet-orange would be 4, 7, and three zeros, or 47,000 ohms, a.k.a. 47 KΩ. Be
careful not to confuse a black third band with a zero; it means no zeros.
132   How to Diagnose and Fix Everything Electronic

           This scheme works great until the resistance value gets below 10 Ω. Try it—there’s
      no way to mark such a value. For resistors that low, the third band is set to gold, meaning
      that there’s a decimal point between the first two bands. So, green-blue-gold-gold would
      be a 5.6 Ω resistor with a ± 5 percent tolerance. You won’t see too many resistors under
      10 Ω, but you might run across one in an audio output stage or a power supply.
           Some resistors have four bands plus a tolerance band. These are higher-precision
      parts with tighter tolerances and must be read slightly differently. With these, read
      the first three bands as numbers and the fourth band as the multiplier. So, the 47 KΩ
      resistor would read yellow-violet-black-red.
           How can you tell what type of resistor you have? Sometimes you can’t. If the
      resistor has a gold or silver tolerance band, assume it’s a standard carbon composition
      part, unless it’s in a very low-noise circuit like an audio preamp, in which case carbon
      film might be a more appropriate replacement. If it has a red or brown tolerance
      band, indicating higher precision, it might be a metal film or metal oxide component.
           Many surface-mount resistors are marked numerically, using the same idea. The
      last number is a multiplier. If you see an R between the numbers, that’s a decimal
      point. You’ll see that only on resistors of rather low value. For instance, 4R7 means
      4.7 ohms. If you see a number with a zero at the end, don’t confuse that to mean a
      number; it indicates no zeros. Thus, 220 means 22 Ω, not 220 Ω, and 220 Ω would
      be marked 221. Also, a letter after the Ω on any type of numerically marked resistor
      is not part of the numerical value, even if it’s a K. So, 47 ΩK is still 47 Ω, not 47 KΩ.
      Used this way, the K denotes 10 percent tolerance. Pretty crazy, huh?
           Some tiny surface-mount resistors are too small for numerical value markings.
      Instead, they sport a two-digit number that must be cross-referenced from a list. The
      number itself has no direct relation to the value. This marking scheme has several
      permutations, and you can find them on the Internet. It’s highly unlikely, though, that
      you’ll ever need to replace one of those tiny resistors, because they carry very little
      current and rarely fail.


      Uses
      Resistors are found in just about every circuit. They limit the current that can pass
      through other parts. For instance, transistors amplify a signal by using it as a control
      for a larger current provided by the power supply, somewhat like the handle on a
      spigot controls a large flow of water. A resistor between the supply and the transistor
      sets how much current the transistor has to control. Without the resistor, the transistor
      would have to handle all of the supply’s current and would self-destruct.


      What Kills Them
      Resistors rarely fail on their own. Heat caused by passing too much current burns
      them out, sometimes literally. Carbon composition resistors can go up in flames or
      become a charred lump when a short in some other part pulls a lot of current through
      them. It’s not uncommon to see one with a burn mark obscuring the color bands.
                Chapter 7       What Little Gizmos Are Made of: Components 133

   Out-of-Circuit Testing
   Use your DMM’s ohms scale to see if the resistance is within the specified tolerance
   range. Most resistors do better than their tolerances, but expect a small difference
   between what’s on the code and what you measure.
       When the resistor is charred beyond your being able to read the color code, it can
   be a real problem unless you have the unit’s schematic diagram. Luckily, resistance
   values follow a standard pattern, because there would be no point in producing
   resistors whose values fell within another resistor’s tolerance range. So, you may be
   able to infer a burned-out band’s value from others that can still be seen. If you ever
   need it, you can look up the list of standard values online.



Potentiometers
   A potentiometer, or pot, is a variable resistor. A small one used for internal circuit
   adjustments is called a trimpot. Pots and trimpots may have two, three or (rarely) four
   leads, but most have three. The outer two are connected to the substrate on which the
   resistive element is formed, with one lead at each end of the resistor. The center lead
   goes to the wiper, a movable metal contact whose point touches the resistive element,
   selecting a resistance value that rises relative to one outer lead while falling relative to
   the other as you turn the knob. See Figure 7-11.
       A two-wire pot has no connection to one end of the resistive element but is
   otherwise the same. Some two-wire pots connect the free end of the element to the
   wiper, which slightly affects the resistance curve as you turn it, but it doesn’t matter
   a whole lot. Two-wire pots are sometimes called rheostats.




                     Figure 7-11     Pots and trimpots
134   How to Diagnose and Fix Everything Electronic

           Four-wire pots, used mostly on stereo receivers to provide the “loudness” function
      which increases bass at low volume levels, are like three-wire pots, but with an extra
      tap partway up the resistive element.
           The amount of resistance change you get per degree of rotation is even from end
      to end on linear taper pots. On log taper pots, also called audio taper, a logarithmic
      resistance curve is used, so that audio loudness, which is perceived on a log curve,
      will seem to increase or decrease at a constant rate as the control is varied. Trimpots,
      whose primary use is for set-and-forget internal adjustments on circuit boards, are
      always linear taper.
           How much power the pot can dissipate depends on its size. It’s not marked on the
      part. In most applications, only small signals are applied to it, so it’s not much of an
      issue.
           Some pots have metal shafts, while others use plastic. Plastic provides insulation in
      applications like power supplies, where you might come in contact with a dangerous
      voltage. Also, some pots have switches built into them, and those may be rotary,
      operating at one end of the wiper’s travel, or push-pull.
           Most trimpots rotate through less than 360 degrees, just like pots. Those used in
      applications requiring high precision may be multi-turn, with a threaded gear inside
      providing the reduction ratio. Multi-turn trimpots use a small screw for adjustment,
      usually off to one side of the body of the component. Be wary of turning the screw,
      because there’s no visual indicator to help you set it back where it was.
           In stereo receivers, pots can be ganged together onto one shaft, so that turning
      it will affect both channels together. Each pot is internally isolated from the other,
      though you may find one end of all of them tied to ground at the terminals.

      Symbols



      Markings
      Pots are numerically marked with their resistance values using the same scheme
      as resistors. A B in the code indicates a linear pot, while a C means a log pot. An A,
      though, can mean either, as the codes have changed over the years. Some pots have
      LIN or LOG printed on them. Most don’t, though.


      Uses
      Pots are used to adjust operating parameters for analog signals and power supply
      voltages. Once, they were the primary method of setting just about everything. In this
      digital age, volume, treble, bass, brightness, contrast and such are more often selected
      from a menu or adjusted with up/down buttons.
           Trimpots on circuit boards are less common too, but you’ll still find some, especially
      in power supplies, including switchers.
                Chapter 7      What Little Gizmos Are Made of: Components 135

   What Kills Them
   Most failures are caused by wear or dirt where the wiper makes contact with the
   resistive element. The symptom is scratchiness in audio, flashes on the screen in
   video, or inability to set the pot to specific spots without the wiper’s losing contact
   with the element. Contact cleaner spray will usually clear it up, but if the element is
   too worn, replacement is the only option.
       Occasionally, the resistive element cracks, resulting in some weird symptoms
   because the wiper is still connected to one end but not the other. Plus, which end is
   connected reverses as you turn the control over the broken spot. Audio may blast
   through part of the control’s range and disappear below the break point.


   Out-of-Circuit Testing
   Test the outer two leads on a three-lead pot with your DMM’s ohms scale. It should
   read something close to the printed value. If the element is cracked, it’ll read open.
        To check for integrity of the wiper’s contact, connect the meter between one
   end of the pot and the wiper. Slowly turn the pot through its range, observing the
   change in resistance. This is one test better done with a good old analog VOM; the
   meter needle will swing back and forth with the position of the wiper, and it’s easy to
   interpret.
        A two-lead pot should read something close to its stated resistance at one end of
   its control range and 0 ohms at the other.
        To verify if a pot is linear taper or log taper, measure the resistance at one-third
   of the rotation and again at two-thirds. See if those values are about one-third and
   two-thirds of the total resistance from end to end. If it’s a log taper part, they won’t
   even be close.



Relays
   Relays are switches controlled by putting current into a coil. When the coil is energized,
   the resulting magnetic field pulls a metal plate toward it, pressing the attached switch
   contacts against opposing contacts. In this way, a small current can control a much
   larger one, just as a transistor does in switch mode.
       Relays may have any number of contacts for switching multiple, unconnected
   circuits at the same time. As with switches, each set is called a pole. Also, some
   contacts may be normally open, or NO, meaning that they are not touching until the
   relay is energized, and some may be normally closed, or NC, meaning the opposite.
   Each direction is called a throw. Thus, a double-pole, double-throw, or DPDT, relay
   would have two switches, each with three contacts: the NO side, the movable contact,
   and the NC side. See Figure 7-12.
136   How to Diagnose and Fix Everything Electronic




                          Figure 7-12      Relays


      Symbols




      Markings
      Relays may be marked schematically for their internal construction—that is, where
      the coil connections are and what kinds of switches are inside. You may find a voltage
      rating; that specifies the coil voltage, not the maximum voltage permitted on the switch
      contacts. A resistance rating is for the coil too. If you find one, you can calculate the coil’s
      pull-in current with Ohm’s Law. It also helps you when testing the coil with your DMM.
           Some relays include an internal diode across the coil to prevent the reverse
      voltage it generates when power is removed from feeding back into the circuitry and
      damaging it. If the relay shows polarity markings (+ and –), it has a diode.
           The maximum current the switch contacts can handle usually isn’t shown, but it
      might be. If the markings read 12 VDC, 3 A, that indicates a 12-volt coil intended to be
      driven with DC power, with switch contacts capable of switching 3 amps. Some relays
      are made specifically for AC coil operation, too, with the appropriate markings.


      Uses
      Relays were once widely used to switch large currents with smaller ones. These days,
      semiconductors usually do that job, but some applications still employ relays. Power
      supply delay circuits, which prevent power from reaching the circuitry for a few
      moments after the supply is turned on, often have relays. Speaker protection circuits
                Chapter 7      What Little Gizmos Are Made of: Components 137

   in high-power audio amplifiers use them too, because the high-current audio signal is
   not impeded at all by a relay, but it would be by a semiconductor.
       Most relays make an audible click when they switch, giving their presence away.


   What Kills Them
   Relay troubles usually involve the switch contacts. Corrosion from age and oxidation,
   and pitting from arcing when large currents are switched, cause resistance or flaky
   contact. Once in awhile, a relay will become sticky, not wanting to open back up
   after power is removed from the coil. This condition can be caused by arcing in the
   contacts making them stick to each other, and by weakening of the spring used to pull
   the plate away from the coil’s iron core.
        If the relay has a removable cover, you may be able to pop it off and clean the
   contacts. Sometimes just pulling a piece of paper soaked in contact cleaner between
   them will spiff them up and restore proper operation. If that’s not enough, very light
   wiping with fine sandpaper may remove the outer layer of gunk. Silver polish works
   too, but be sure to get it all off when you’re done. Be careful not to bend the contacts,
   and don’t sand off the plating; it’s vital for their long-term survival. Whichever method
   you use, wipe the contacts with cleaner-soaked paper to remove residue before you
   put the relay’s cover back on.


   Out-of-Circuit Testing
   Use your DMM’s continuity or lowest ohms scale. Check for coil continuity. If there’s
   a diode, be sure to check in both directions. The coil shouldn’t read 0 ohms; there’s
   enough wire there for dozens to a few hundred ohms. If it reads very near zero on
   a relay that has a diode, suspect a shorted diode, especially if the symptom suggests
   that the coil doesn’t want to pull in the switch. Also check that there is no continuity
   between the coil and its metal core. If there is any, you need a new relay.
        Check the NC contacts with the meter. They should read 0 ohms or very close
   to it. Use your bench power supply to energize the coil. If it has a diode, be certain
   to get the polarity correct, with + to the diode’s cathode, not its anode. (Remember,
   the diode is supposed to be wired backward, so it won't conduct when power is
   applied.) With no diode, polarity doesn’t matter. Once you hear the click, check the
   NO contacts. They should also read 0 ohms or very close to it. When you disconnect
   the power supply, the contacts should return to their original state. The NO contacts
   should open and the NC contacts should close.



Switches
   Switches permit or interrupt the passage of current. There are many, many kinds of
   switches, and they’re used in just about everything. Toggle switches, slide switches,
   rotary switches, leaf switches, pushbuttons, internal switches on jacks…there’s
   practically no end to the varieties. See Figure 7-13. Many newer products do not
138   How to Diagnose and Fix Everything Electronic




                       Figure 7-13     Switches



      have “hard” power switches that actually disconnect power to the unit. Instead, a
      low-current switch signals the microprocessor, which then shuts down power using
      semiconductors to interrupt the flow.
           Switches can have any number of contacts for switching multiple, unconnected
      circuits at the same time. Rotary and slide switches, especially, may have many
      sets, while toggle switches rarely have more than two or three. Each set is called a
      pole. Also, some contacts may be normally open, or NO, meaning that they are not
      connected with the switch in the “off ” position, and some may be normally closed,
      or NC, meaning the opposite. Each direction is called a throw. Thus, a double-pole,
      double-throw, or DPDT, switch would have two separate sets of contacts, each with
      three elements: the NO side, the movable contact, and the NC side.

      Symbols




      Markings
      If marked at all, switches may show their maximum voltage and current ratings.


      Uses
      Expect to find switches everywhere. From pushbuttons on front panels and remote
      controls to tiny slide switches on circuit boards, they handle power and information
      input in essentially all products. Leaf switches, with bendable, springy metal arms,
                 Chapter 7      What Little Gizmos Are Made of: Components 139

    sense the position of mechanisms like tape transports and laser optical heads, informing
    the microprocessor of the state of moving parts. Switches inside jacks sense when
    accessories are plugged in, altering system behavior to accommodate them.


    What Kills Them
    As with relays, age, oxidation and contact pitting from arcing usually do them in.
    If the switch’s construction permits any access, try spraying some contact cleaner
    inside, and then work the switch a bunch of times.


    Out-of-Circuit Testing
    Test switches with your DMM’s continuity or lowest ohms scale. The contacts should
    read 0 ohms when closed and infinity when open. There’s an exception, though:
    Some pushbuttons, such as the kind on remotes, laptop keyboards and tiny products
    like digital cameras, use a carbon-impregnated plastic or rubber contact to make the
    connection. You can identify them by their soft feel when pressed; they don’t click.
    These switches are intended only for signaling, not for handling significant current,
    and they may have a few tens of ohms of resistance when in the “on” state. While
    such a reading would indicate a bad toggle switch, it’s fine with these little guys.


Transistors
    Along with integrated circuits, transistors are the active elements that do most of the
    work in circuits, amplifying, processing and generating signals, switching currents and
    providing the oomph needed to drive speakers, headphones, motors and lamps. See
    Figure 7-14.




                     Figure 7-14     Transistors
140   How to Diagnose and Fix Everything Electronic

           Transistors act like potentiometers, but instead of your hand’s turning the shaft, a
      signal does. The current fed by the power supply through the pot can be much greater
      than that required to turn the shaft, providing gain, or amplification, as the wiggling
      signal molds a bigger version of itself from the power supply’s steady DC.
           There are thousands of subtypes of transistors, but most fall into three categories:
      bipolar, JFET and MOSFET.
           Bipolar transistors are the standard types used in products since the 1950s. They
      come in two polarities, NPN and PNP, and consist of three elements joined by two
      junctions. The three elements are the base, emitter and collector, each with its own
      lead. Current passing between the base and the emitter permits a much larger current
      to pass between the collector and the emitter, with one of them being fed from the
      power supply. The greater the voltage difference between the base and emitter, the
      more current will pass, and a proportionally higher current can pass between collector
      and emitter. In most configurations, the signal is applied to the base, causing a bigger
      version of itself to be formed from the flow between the collector and emitter. The
      ratio of base-to-emitter current to collector-to-emitter current is the transistor’s gain,
      and is inherent in the component’s design.
           In an NPN transistor, the base must be positive with respect to the emitter for
      collector-to-emitter current to pass. In a PNP transistor, the base must be negative. So,
      the two types are of opposite polarity and cannot be interchanged. Most transistors
      used today are NPN, but you will find circuits with some PNP parts.
           JFETs, or junction field effect transistors, work on a similar principle. They label
      their three elements differently. Instead of the base, the controlling terminal is called
      the gate. The emitter is called the source, and the collector is the drain.
           Instead of a current passing from gate to source, application of a voltage to the gate
      goes nowhere but results in an electric field that controls a channel in the transistor,
      permitting current to pass between drain and source. This gives the JFET a very high
      input impedance, which is another way of saying that it does not take much signal
      current to turn it on.
           MOSFETs, or metal-oxide-semiconductor field-effect transistors, are similar to JFETs,
      and they use the same terminal names, but their internal construction is a bit different.
      They have even higher input impedance and some other desirable characteristics that
      have resulted in their pretty much dominating FET applications. You may see JFETs
      in older gear, but you’re much more likely to see nothing but MOSFETs in newer
      equipment.
           Like bipolars, FETs come in two polarities, P-channel and N-channel, corresponding
      to PNP and NPN bipolar transistors. They also come in enhancement mode and depletion
      mode types, specifying what happens when the gate voltage is zero. An enhancement
      mode FET will be turned off with no voltage at the gate; like a bipolar transistor, it
      requires a bias voltage to turn it on. A depletion mode FET will be turned on with zero
      gate voltage. The only way to turn it off is to apply a voltage of opposite polarity to the
      one that will increase current flow.
           Luckily, you don’t need to worry too much about these arcane details. If a FET is
      bad, you’ll look up its part number and replace it with a compatible type. Still, knowing
             Chapter 7        What Little Gizmos Are Made of: Components 141

the basics of how these parts work is essential for understanding how to troubleshoot
circuits using them…which is pretty much all circuits.
    As you can see, transistors have many parameters, so it’s not surprising that
there are thousands of subtypes with different gains, power dissipation capabilities,
frequency limits and so on. Some are similar enough that they can be interchanged
in many circuits, but most are not. To replace one part number with another, you
need a transistor substitution book or an online cross-reference guide.

Symbols

                 C                E                   D                   S
        B                 B
                                                                G
                                            G
                 E                C                   S                   D
             NPN               PNP              N-channel           P-channel
                                                  JFET                 JFET


                 S                D                  S                   D

        G                                 G
                         G                                  G

                 D                S                  D                   S
           P-channel       N-channel        P-channel         N-channel
          MOSFET enh      MOSFET enh       MOSFET dep        MOSFET dep




Markings
Transistors are marked by part number, called a type number. There are thousands
of these numbers! Some numbers indicate whether a bipolar part is NPN or PNP.
If the number starts with 2SA or 2SB, it’s PNP. If it starts with 2SC or 2SD, it’s NPN.
Sometimes the 2S will be left off, and there are plenty of type numbers that don’t
follow this scheme at all. A 3N indicates a FET, but some of them have numbers
starting with 2N, and there are lots of other kinds of numbers for these too.
     Some transistors have house numbers, which are proprietary numbering schemes
used by different manufacturers to mean different things. There is no way to ascertain
what the industry-standard number would be for such a part. Tiny, surface-mount
transistors often have no numbers at all.
     The arrangement of the leads varies with transistor type. Small Japanese parts
with leads are usually laid out ECB, left to right, while American parts are often EBC.
Metal-encased power transistors have only two leads and use the metal casing as the
collector. Plastic power transistors are usually BCE, with the metal tab, if there is one,
being C.
142   How to Diagnose and Fix Everything Electronic

      Uses
      In discrete (nonintegrated-circuit) stages, transistors are the active elements doing the
      work, with passive parts like resistors and capacitors supporting their operation. You’ll
      see this kind of construction in radio receivers, audio amplifiers and some sections of
      many other products. In stages where an IC is at the center of the action, transistors
      frequently do the interfacing between the IC and other parts of the circuitry, especially
      areas requiring more current than an IC can supply. MOSFETs are used as switches,
      permitting the microprocessor to turn power on and off to various parts of the circuitry.
      Some of their very sensitive varieties are used to amplify and detect radio signals. Many
      audio power amplifier output stages are made from bipolar transistors. It’s hard to find
      any function that transistors don’t do. After electrolytic capacitors and bad connections,
      transistors will be the focus of much of your repair work.


      What Kills Them
      Transistors are not especially fragile, but they work hard in many circuits and fail
      more often than do most components. Overheating due to excessive current will burn
      them out, as will too high a voltage. MOSFETs are particularly prone to shorts from
      static electricity. Sometimes the internal structure of a transistor develops a tiny flaw,
      and the thing self-destructs with no apparent cause. In fact, many random product
      failures occur for precisely this reason. You change the part and the unit works again,
      and you never find any reason for the dead transistor.


      Out-of-Circuit Testing
      If you have a transistor tester, use it! Nothing’s easier than hooking up the leads and
      getting the test result. If you don’t have one, you can use your DMM’s diode test or,
      lacking that, the ohms scale to check for shorts. If you get near-zero ohms between
      any two leads, check in the other direction. If it’s still near zero, the part is shorted.
           Checking for opens is a bit more complicated, because some combinations of
      terminals should look open, depending on to what the control terminal is connected.
      Connecting the base of a bipolar transistor to its collector should result in its turning
      on, showing measurable resistance between the emitter and collector in one direction.
      Connecting the base to the emitter should turn it off.
           Similarly, connecting the gate of a FET to its drain should turn it on, and connecting
      it to the source should turn it off. However, some FETs are symmetrical and will turn
      on with the gate connected to either of the other terminals, as long as the polarity of the
      applied voltage is what the gate needs. And the whole situation is complicated by the
      enhancement/depletion mode issue, because a depletion mode FET will stay on. FETs
      are not easy to test with an ohmmeter!
                Chapter 7       What Little Gizmos Are Made of: Components 143


Voltage Regulators
   Voltage regulators take incoming DC and hold their output voltage to a specific value as
   the incoming voltage fluctuates or the load varies with circuit operation. Yesteryear’s
   simple products often had no voltage regulators, but practically everything made
   today does.
       Linear regulators act like automatic variable resistors, passing the incoming
   current through a transistor called the series pass transistor and setting the base
   current to keep the output voltage constant. When the load changes and the voltage
   increases or decreases, the regulator detects that and adjusts the base current to
   compensate, altering the transistor’s resistance and permitting more or less current
   to go through it. The power lost in the resistance of the transistor is wasted and
   dissipated as heat.
       Switching regulators chop the incoming current into fast pulses whose width can
   be varied. Those pulses are then applied to a capacitor, which charges up, converting
   the pulses back to smooth DC. By monitoring the output voltage, the regulator
   detects changes and alters the pulse width. The wider each pulse, the more current
   can charge the capacitor, raising the output. Narrower pulses lower it. While more
   complicated, this approach, called pulse-width modulation (PWM), supplies only
   the current required to keep the output voltage constant, without wasting the excess
   as heat. Thus, it is much more efficient.
       PWM regulation is an inherent feature of switching power supplies, and some
   products use switching regulators internally as well. They are complicated, though,
   and also generate a fair amount of RF noise, so they aren’t suitable for all uses.
       Linear regulation, while wasteful, is still very common in low-current applications
   because the amount of power wasted is trivial. The linear approach is a lot simpler and
   cheaper, too, making it attractive.
       While both types of regulators once took a bunch of components to implement,
   they can be had in chip form today, requiring just a few external parts to support their
   functions. The three-terminal linear voltage regulator, available in both fixed- and
   variable-voltage varieties, is the most common type you’ll find. It looks like a transistor
   but is really an IC, with one terminal for input, one for ground and one for output.
   See Figure 7-15. Hang a capacitor or two on it and it’s ready to rock. Some products
   have several regulators supplying separate sections of their circuitry.

   Symbol
                IN    OUT
                     REF



   Markings
   Voltage regulators are marked by part number, like transistors. Some standard markings
   tell you the voltage, which is handy. In particular, the widely used 7800/7900 series of
   linear regulators offers useful marking information. All regulators starting with 78 are
   positive regulators with negative grounds. A 7805 is a 5-volt regulator, a 7812 is 12 volts,
144   How to Diagnose and Fix Everything Electronic




                       Figure 7-15     Voltage regulators


      and so on. All parts starting with 79 are negative regulators with positive grounds. They
      use the same voltage numbering scheme.
          Other regulators don’t necessarily indicate their voltages in the part number, and
      you will have to look them up.


      Uses
      Voltage regulators provide stable voltage to entire devices or sections of them. In
      some applications, a regulator may feed a single stage or area of the circuitry. Some
      switching regulators can boost the voltage and regulate it at the same time. This is
      especially useful in battery-operated devices employing only one or two 1.5-volt cells
      but requiring a higher voltage.


      What Kills Them
      Pulling too much current through a regulator can overheat and destroy it. This is
      especially true with linear regulators. Voltage spikes can cause internal shorts, and
      random chip failures occur too. Switching regulators are prone to blowing their chopper
      transistors, just like switching power supplies.


      Out-of-Circuit Testing
      You can use the ohms scale to check for shorts, but that’s about it. To evaluate
      a regulator properly, it needs to be in a circuit, receiving power.
                Chapter 7      What Little Gizmos Are Made of: Components 145


Zener Diodes
   Zener diodes are special diodes used in voltage regulators. In the forward direction,
   they conduct like normal diodes. In the reverse direction, they also act like regular
   diodes, blocking current. When the reverse voltage rises above a predetermined value
   set in manufacture, the zener breaks down in a nondestructive manner and conducts.
   This results in a constant voltage drop across the part, making it useful as a voltage
   reference. Zeners dissipate power as heat, so they are rated in watts for how much
   they can take before overheating and burning out.
        Zeners look much like other diodes, but many have somewhat beefier cases and
   thicker leads to increase heat dissipation capability. See Figure 7-16.

   Symbol


   Markings
   The band on a zener’s case indicates the cathode, as with any diode. Since zeners are
   used for their reverse breakdown action, though, expect them to seem to face the
   wrong way in the circuit, with the band connected to positive. In fact, that’s one way
   to help determine whether a diode you see on a circuit board is in fact a zener, and
   not just a normal diode.
        Zeners are marked with part numbers, when they are marked at all. If the number
   begins with 1N47 and is followed by two more digits, that’s a zener. Some have numbers
   like 5.1 or 9, which would seem to suggest their breakdown voltages. This is not always
   the case! Look up the numbers to determine a zener’s characteristics.


   Uses
   Zeners provide a stable voltage reference in many circuits, especially power supplies
   and regulators. Even switching regulators may use them for reference.
       Zeners can be used as linear regulators by themselves when only a small current
   supply is required. A resistor will be used to limit the current going to the zener, and




                    Figure 7-16     Zener diodes
146   How to Diagnose and Fix Everything Electronic

      the regulated voltage will appear at the cathode, while the anode goes to ground (in
      normal, negative-ground circuits).


      What Kills Them
      Putting too much current through a zener will exceed its wattage rating and overheat
      it, destroying the part. Over time, even zeners in proper service may fail from the
      cumulative effects of heating.


      Out-of-Circuit Testing
      Test zeners for basic diode operation using the diode test or ohms scales on your
      DMM. Zeners can short, but most fail open, or at least they appear to do so. In fact,
      they may short and pass so much current that they melt inside, quickly opening.
            If the zener tests bad as a normal diode, it is bad. If it tests good, it may have lost
      its breakdown ability and still be bad, though. There’s an easy way to tell, using your
      bench power supply. For this to work, the supply must be able to deliver a voltage at
      least a volt or so higher than the zener’s expected breakdown voltage.
            Take a 10 KΩ resistor (brown-black-orange) and put it in series with the zener’s
      cathode, using clip leads. Connect your bench power supply with its + terminal to the
      other end of the resistor and its – terminal to the anode of the zener. Set your DMM
      to read DC volts and hook it across the zener. Turn the bench supply as low as it will
      go, and then switch it on. Increase the supply’s voltage while watching the DMM. As
      the indicated voltage rises, it should hit the zener’s breakdown point and the DMM’s
      reading should stop rising, even though you continue to crank up the power supply.
            If the voltage keeps going up past the zener’s breakdown voltage, the part is bad.
      If it stops very near the rating, it’s fine; standard zeners are not high-precision devices
      and may be off by a few fractions of a volt.
            This test is also handy for characterizing unmarked zeners, as long as they are
      good. When you encounter a dead unmarked zener, determining what its breakdown
      voltage was supposed to be can be a real problem, unless you can find a schematic
      diagram of the product. Sometimes you can infer the breakdown voltage from other
      circuit clues. We’ll get into that in Chapter 11.
            There are many other kinds of less frequently used components. If you run across
      one that doesn’t fit into any of the categories we’ve discussed, look up its part number
      to find out what the part does. An online search will usually turn up a data sheet
      describing the component in great detail.
Chapter                 8
 Roadmaps and Street Signs:
 Diagrams

 T   oday’s products can contain hundreds or even thousands of components. Even
     after you’ve considered a unit’s failure history and pondered a preliminary
 diagnosis, there can still be lots to examine and test. How the heck do you find your
 way around what looks like a city on Mars?
     There are three types of roadmaps to help you navigate the innards of an
 electronic device:

  •	 Block diagram This lays out the device by the function of each section and
     its basic interconnections. It does not show individual components or specific
     connection points. It’s the most general, conceptual view, analogous to a map
     showing cities and route numbers for major highways between them, but not
     street-level detail. See Figure 8-1.
  •	 Schematic diagram This shows all the components and interconnections but
     does not indicate their purposes by specifying sections or overall structure. This is
     the street-level map. See Figure 8-2.
  •	 Pictorial diagram This uses drawings of the parts and shows their
     interconnections. The pictorials included in service manuals are really layout
     diagrams, detailing the placement of components as they exist on circuit boards and
     chassis. This is the drawing of landmarks and where to find them. See Figure 8-3.

     All three diagrams work together to guide you to your destination. The block
 diagram helps you grasp the signal flow and interactions between circuit sections so
 you can see how they are supposed to work with each other. The schematic shows
 you individual components and stages so you can zero in on specific components you
 may want to scope or pull for testing. The pictorial helps you find the darned things!
     If you can obtain only one style of diagram, get the schematic, because it offers
 the detail necessary for troubleshooting at the level of individual components. In
 years gone by, most products included a schematic, printed inside the case or in the
 instruction booklet. That became impractical as gadgets got more complex; there were

                                                                                    147
148   How to Diagnose and Fix Everything Electronic

                                            MIC                                                           EFM
                         Analog                                            ADC                                                  Digital
                                           AMP                                                           Encoder
                           In                                              U 210                                                 Out
                                           Q 203                                                          U 302




                                                                                                                   LCD
                                                                          System
                                                                          Control
                                                                           U 801
                                                Keypad
                                                                 +5 V DC
                                           DC–DC
                                            Conv
                                    Batt. Q 110,111

                                                                 –5 V DC

                          Figure 8-1            Block diagram


      just too many parts to fit the diagram in such a small space. For awhile, manufacturers
      supplied fold-out schematics with their instruction booklets, but they finally began
      omitting the sheets as products no longer had incandescent lamps, snap-in fuses or
      any other parts the user could change. After all, only a service tech could really make
      use of diagrams anyway, so why spend the money to print them by the hundreds of
      thousands? Instead, service manuals were made available to repair shops, and the


                                            R 212 47 K

                 R 210                                                   R 213
                 4.7 K                      2                            10 K                           9
                                                –          1                                               –            8
                                            3 +                                                         10 +
                                                    U 201 A                                                    U 201 B
               C 215        C 216   R 211                               C 217      C 218
                                                     TL084                                                      TL084
                .01          .01    22 K                                 .01        .01

                                                    C 219 .01
                                                                                                                                             C 225
                                                                                                                               Q 201
                             R 214 5.6 K                                                                                                      .22
                                                                                                                              2N 3906
                                                                 R 216                         11
                                       13                                                  6            U 201 D                     C 224
                                                                 2.2 K
              R 220                         –                                                   –                                    .001
                                                      14                                                   7
              10 K                     12 +                                                5 +
                                                                                                          TL084
                                                U 201 C                                             4
                            C 220   R 215        TL084          C 221      C 222     R 217
                             .01    68 K                         .01        .01      470 K                     C 223                        R 219
                                                                                                                   .1       R 218
                                                                                                                                            33 K
                                                                                                                            22 K

      C 226
       .001




       Figure 8-2           Section of a schematic diagram
                      Chapter 8      Roadmaps and Street Signs: Diagrams 149




         Figure 8-3     Pictorial diagram


end user, soon to be called the “consumer,” was left high and dry, no longer privy
to the products’ insides. “No user-serviceable parts inside. Refer service to qualified
personnel,” replaced the diagrams. Is it any wonder they call today’s gadgetry
“consumer electronics”?
     Service manuals were cheap and plentiful, and shops kept huge rows of filing
cabinets bursting with them. In addition to manuals generated by the products’ makers,
the Howard W. Sams company produced its own comprehensive line of Photofact
schematics for just about everything out there. If you couldn’t get a schematic from
Zenith, you could get a Sams easily enough for a buck or two.
     As products got still more complex, manuals grew from a few pages to a few
hundred, with large, fold-out schematics and very detailed, color pictorials. Producing
these big books became quite expensive, so their prices skyrocketed. Shops continued
to buy them—they had little choice—but no consumer would spend more for a
manual than the product cost in the first place! Companies gradually reduced and
finally abandoned the infrastructure for selling manuals to the public, and today’s age
of “use it, wear it out and toss it” was in full swing. Many manufacturers will no longer
sell schematics or service manuals to consumers, thanks in part to fear of potential
150   How to Diagnose and Fix Everything Electronic

      lawsuits by injured tinkerers. Some companies won’t even sell manuals to service
      shops unless they’re factory-authorized warranty service providers. And, believe it
      or not, some even refuse to provide diagrams to those facilities! Secretive computer
      makers, in particular, only let authorized servicers swap boards; the techs work on
      their machines for years without ever seeing a schematic of one.
           Where does this leave you? Forget calling the major manufacturers; they won’t
      sell you a service manual no matter how you plead. There are online sources, though,
      continuing to provide this vital information, at least for some products. Howard
      W. Sams continues as Sams Technical Publishing, at www.samswebsite.com. Their
      Photofact and Quickfact manuals aren’t $1.50 anymore. As of this writing, most of
      them cost $20 and up. Still, for a tough case that has you going around in circles, it
      may well be worth the investment. Numerous other sites offer diagrams, some for
      free. Doing a Web search may turn something up, and it’s always worth a try.



Hooked on Tronics
      Reading a schematic is a bit like reading music: learning to name the notes is just the
      beginning. To really understand what’s going on, you need to recognize the larger
      harmonic and rhythmic structures and how individual notes fit into and connect
      them. Identifying components on a schematic is a good start, but seeing how they
      form stages and sections, and how those work with each other, is vital to being able
      to find the ones that aren’t properly performing their functions. The best techs have
      a good grasp of circuit fundamentals, but there’s no need to be an engineer or a math
      whiz. It’s far more useful to be familiar with the overall structure and with how basic
      circuit elements like transistors work.
           In Chapter 7, we reviewed the component symbols for the most common parts.
      Along with those, schematics include symbols for other items. Here are some you’re
      likely to encounter:

      AC Voltage     The presence of AC voltage is indicated by a sine wave.

      Antenna The antenna symbol represents any type of antenna, even if it doesn’t
      resemble the symbol.

      Battery In addition to the main batteries powering portable devices, small backup
      batteries may be found on circuit boards, either soldered or in holders. When wires
      connect batteries or battery holders, it is standard to use a red wire for positive and a
      black wire for negative.                                                             –    +


      Conductors, Joined This is where two wires or circuit traces meet and connect.

      Conductors, Not Joined This is where two wires or circuit traces cross on the
      schematic (but not necessarily physically in the device) without connecting.
      Older schematics use the 3D-looking loop shown on the right. Newer diagrams
                      Chapter 8        Roadmaps and Street Signs: Diagrams 151

don’t, because today’s products are so densely packed with
interconnections that the loops become unwieldy and take up
too much room on the schematic. Remember, if there’s
no dot where they cross, they are not connected! When a
conductor meets another one without crossing, though, it is
connected whether or not there's a dot.                             (a)          (b)


Conductors, Merged This shorthand description of multiple wires is
widely used in digital gear, particularly when parallel data lines all go to
the same chip. Instead of showing a separate conductor for each line, only
one is shown, making complex schematics a little less cluttered and a bit easier to read.

DC Voltage The lines represent what DC looks like on an oscilloscope, with the
dotted line indicating ground.

Ground There are four types. Earth (at left in the illustration) indicates
a connection to the AC line’s ground lug. Chassis (at right) means the
connection goes to the unit’s metal chassis or, lacking one, a common
point on the circuit board. The earth symbol is often used in place of the chassis
ground symbol—you’ll see it in battery-operated gear that never gets connected to
the AC line—but not the other way around.
     Analog and digital ground symbols are used in devices having separate
ground points for their analog and digital sections. This arrangement helps
                                                                                       A
keep electrical noise generated by the digital system from intruding into
sensitive analog circuits. Many CD, DVD and MP3 players have separate
analog and digital grounds. In some products, the grounds meet, and only
the length of a circuit board trace separates them. In others, analog and
                                                                                       D
digital grounds remain separate, and connecting them externally will cause
malfunction or undesired noises in the output signal.

Jack There are many styles of jacks, so jack symbols are somewhat pictorial. Also,
some jacks have internal switches that sense when a plug is inserted, and those will
be shown too.




                          –        +




Speaker The speaker symbol looks like a classic loudspeaker, but it can
be used to indicate headphones as well. Sometimes a drawing of a headset
will be shown instead.
152   How to Diagnose and Fix Everything Electronic

      Call Numbers
      Each component will have a part number unique to that product’s schematic. Some
      techs refer to it as the call number. That number is unrelated to the part number
      printed on the component, the one by which you can look up the part and learn its
      electrical characteristics. Instead, the call number is derived from the parts list found
      in the service manual.
           Each call number begins with a letter specifying the type of component, followed
      by a few numbers. The first number tells you in what section the component resides,
      and the others are unique identifiers. Designations like R201, L17 and Q158 are call
      numbers.
           The section number is arbitrary and varies from product to product, but parts with
      the same first number will live in the same neighborhood. For example, R201, Q213
      and C205 will all be in the same area, but R461 and Q52 won’t. And C206 is probably
      right next to C205, or at least not very far away. If the circuit board is labeled, it’ll
      show those numbers next to each component.
           Every part normally has its own call number, but multisection integrated circuits
      such as op-amps may be shown with each section as a separate device, even though
      they’re really in one package. In Figure 8-2, the TL084 chip has four sections, each
      labeled A, B, C or D. On another schematic, the same component could be drawn as
      one rectangle resembling the shape of the real thing, with all four sections inside. In
      that case, each section’s triangular op-amp symbol might be drawn inside the little
      box, but you can’t count on that. Presenting the part as separate sections helps keep
      the signal flow clearer and is the preferred method.
           Although each schematic has unique call numbers, the component type letters
      are somewhat standardized. Here are the letters in common use:

      Component                  Designator               Component              Designator
      Capacitors                 C                        Relays                 RL
      Connectors                 J or CN                  Speakers               SP
      Crystals and resonators    X or Y                   Switches               S or SW
      Diodes                     D                        Transformers           T
      Fuses                      F                        Transistors            Q
      Coils (inductors), but     L                        Test points (places    TP
      not transformers                                    to put your probes)
      Integrated circuit chips   IC, U or Q               Voltage regulators     IC, U or Q
      Resistors                  R                        Zener diodes           Z, ZD or D
      Potentiometers             R or VR

           Because Q is the designator for transistors, it gets used for just about anything
      made from them, even if they are microscopic structures in an integrated circuit chip.
      Newer schematics, though, are more likely to differentiate, calling integrated circuits
      U, transistors Q, and voltage regulators IC.
                         Chapter 8      Roadmaps and Street Signs: Diagrams 153

        Call numbers are handy even if you don’t have a parts list, because they help you
   identify mystery components. Especially in this age of ultra-tiny, surface-mount parts,
   some look so similar that it’s hard to guess what they are. If you see a call number
   starting with an R, you know the part is a resistor. An L tells you it’s an inductor, and
   so on. Should you run into a part with a designator not covered in this book, a quick
   trip to the Internet will turn up its meaning.



The Good, the Not Bad, and the Miserable
   Not all schematics are alike. There are good ones, even great ones. There are average
   ones. And there are the dreadful diagrams that are almost worse than none at all.


   The Good
   A good schematic is logically laid out, showing most stages with signal flow from
   left to right, with enough space between the stages to make the organization clear. It
   includes call numbers and part numbers, with resistor and capacitor values specified.
   A really good one may have arrows indicating signal flow through and between
   stages. A truly great diagram even has voltage readings and—it doesn’t get better than
   this—snapshots or drawings of scope waveforms at various test points! If you’re lucky
   enough to work with such a schematic, it’ll greatly speed up your hunt. Touch a probe,
   compare what you see to the diagram, and either it looks the same or it doesn’t. In
   real life, it’s rarely as simple as that, but having those guideposts is a wonderful help.


   The Not Bad
   A merely okay diagram is clear, with a reasonable sense of organization. It has call
   numbers but probably no part numbers or parts values. Forget about signal flow
   arrows or waveforms. It’s no GPS, but it’s a serviceable roadmap. Everything is
   accurate and nothing is left out. Which brings us to the dark side, that malevolent
   maw of misleading misery, the incorrect schematic.


   The Miserable
   A really bad schematic may have reversed diode polarity, wiring errors, incorrect
   connection indications on conductors crossing each other, or omission of some parts,
   any of which can confuse the living heck out of you and send you off in the wrong
   direction. Switches show no indication of what they do in what position, and the
   drawing might not even be clear enough that you can read parts of it. Still, even a
   miserable schematic can be better than none, as long as you remember not to trust
   everything you’re seeing. Occasional errors crop up in even the best diagrams, of
   course, but it’s rare to find a truly rotten schematic from a major manufacturer. I’ve
   seen some doozies from off-brand companies, though.
154   How to Diagnose and Fix Everything Electronic


Once Upon a Time…
      To get started reading schematics, consider the organization of a book. It begins with
      letters that form words, which make sentences. Those are grouped into paragraphs
      and finally into chapters. Each paragraph links with the others to tell a story, and
      the chapters present a progression driving toward the finish. Some conditions are
      introduced at the start of the book and resolved at the end.
           Electronic devices are organized much the same way. Components work together
      to form stages, each one feeding others, resulting in a signal flow proceeding from
      some starting point, such as a microphone, antenna or DVD, to some ending point,
      perhaps a display screen or a speaker. Each stage performs a function contributing to
      the overall processing. A group of stages involved in a particular part of the device’s
      operation constitutes a section dedicated to a specific purpose.
           With occasional exceptions, a schematic’s signal flow in each stage proceeds from
      left to right. Signal flow between stages normally goes left to right as well. So, the most
      sensitive stages handling the weakest signals are usually on the left side of the page.
      If you’re looking for an antenna or microphone input, look for it there. In a power
      supply, the AC line connection is probably on the left side, too, as it’s considered the
      supply’s input. Very complex schematics sometimes violate these conventions, simply
      because they run out of room on the page.
           Look for power supply sections at the bottom of the page. Output sections,
      LCD screens and speakers should be on the right. Processing stages, such as the IF
      (intermediate frequency) stages of a radio receiver or the microprocessor in an MP3
      player, will be in the middle.
           When reading a schematic, keep your eye on the story and its central characters.
      Not all players are equally important. The plot is driven by the active elements like
      transistors and ICs, since they do most of the work. Crystals and resonators generate
      signals, so they’re crucial characters, without which the story never gets moving.
      After those, look for coupling elements such as transformers, capacitors and resistors
      linking one stage to the next. They move the plot forward, because signals will be
      flowing through them on the way to subsequent stages. The other resistors, capacitors
      and coils set the voltages, currents and various conditions the big shots need to do
      their jobs. Those subplots are necessary to the overall story but not central to its
      theme. Try not to let them distract you from the primary action. Let’s look at the
      schematics of a few circuits and how to interpret them.


      Amplifier Stage
      For our first adventure, we’ll look at a single stage. Let’s examine every part in it, what
      it does and what would happen if it malfunctioned.
           This one is an inverting amplifier, typical of what you might find in just about any
      product. See Figure 8-4. Inverting means that the output signal rises as the input signal
      falls, and vice versa, producing a replica that’s upside down. Sometimes inversion is
      necessary to the circuit’s operation, while other times it’s just an irrelevant consequence
                      Chapter 8         Roadmaps and Street Signs: Diagrams 155

                                                      +V



                                                           To next
                                            C3              stage
                                   R1


                                                 C         TP1
                                           Q1
                                          B
                      Input
                              C1
                                   R2            E

                                                           C2
                                                 R3



                       Figure 8-4       Amplifier stage



of getting voltage gain from the stage. Either way, you’ll see lots of inverting amplifier
stages.
      The stage has four major points where things go in and out. At the top, power
is applied through the transformer, reaching the collector of the transistor. Signal
input is on the left and goes to the base of the transistor, Q1, via C1. The output is at
Q1’s collector, which is why TP1, the test point, is there. Finally, R2, R3 and C2 go to
ground. Ground is as important an input/output point as the others; without ground,
you’ve got a paperweight.
      To be a sharp troubleshooter, you need to understand how the circuitry is
supposed to function. So, let’s see how this thing works, starting at the top. Power goes
through the winding of the transformer and reaches the collector of the transistor, Q1.
It’s the active element, so it’s the central character. When the transistor is turned off,
no current passes through the transformer, because the lower end of its winding sees
no connection to ground. When the transistor is turned on, the path between collector
and emitter connects, effectively grounding the transformer’s winding and completing
the circuit, pulling current through the winding.
      It’s not an all-or-nothing proposition, though. Remember, transistors act like
variable resistors (potentiometers), except that signals, rather than your fingers, turn
them up and down. Here, the signal is applied to Q1’s base through C1. C1 couples
only the AC component of the incoming signal to Q1, preventing any DC in the signal
from reaching the transistor, and also preventing any DC from Q1 getting back into the
previous stage.
      Where R1 and R2 meet establishes a voltage somewhere between the power
supply’s value and ground, to bias the transistor’s base, or put a little DC on it,
keeping it turned on through whatever portion of the incoming signal’s waveform
156   How to Diagnose and Fix Everything Electronic

      the amplifier is intended to amplify. As with most amplifiers, we want the whole
      waveform, so the transistor has to be biased with enough positive current that when
      the incoming signal goes negative, the transistor’s base never gets below about 0.6
      volts, which is the cutoff point for a standard bipolar silicon transistor. The bias
      current has to flow through R2, so its value determines how much is available to the
      transistor’s base; the higher R2’s resistance, the less bias current there will be.
           The transistor passes current from collector to emitter in proportion to how much
      current passes from base to emitter, as shown by the arrows. The ratio of the two
      currents determines how much the transistor can amplify a signal. It is an inherent
      part of the component’s design and is given in its specifications. As the incoming
      signal wiggles up and down, the base current varies with it, causing the transistor
      to pull a proportionally larger current through the transformer on its way to R3 and
      finally ground. That forming of the power supply’s DC into an enlarged replica of
      the signal is called gain and is the essence of amplification and the foundation of all
      modern electronics, analog or digital.
           Ah, R3 and C2. What’re they there for? R3 limits the total current through the
      circuit; without it, the transistor would attempt to pull the power supply’s entire
      current capability to ground, dragging down the supply and probably blowing the
      transistor or the transformer. R3 limits the total base current as well, because it’s in
      series with that path too. Thus, it sets a limit to how much signal current the previous
      stage has to supply.
           C2 is a little trickier to explain. Transistors, having adjacent regions of
      semiconductor material in them that are not at the same voltage at the same time,
      also behave like capacitors. They store some charge and take a little time to discharge.
      The presence of R3 slows that down because the discharge has to reach ground
      through its resistance. This forms a time constant, which is a fancy way of saying that
      there’s an upper limit to how fast the transistor can get rid of its charge. The bigger
      the value of R3, the longer the discharge process takes, and the slower the transistor
      can react to incoming signal changes. When the frequency of the incoming signal is
      faster than the time constant, the transistor’s residual charge fills in as base current
      drops, acting like any filter capacitor and smoothing out the waveform. As a result,
      the transistor can’t react quickly enough to respond and amplify the signal. Thus, the
      upper speed limit, or frequency response, of the amplifier drops off.
           C2 allows the rapidly changing parts of the signal to reach ground with less
      resistance, discharging the transistor faster. The apparent resistance of a capacitor
      drops with increasing frequency, because it never gets the chance to charge fully and
      oppose the incoming current. So, C2 compensates for the transistor’s capacitance,
      giving it a lower-resistance path to ground with increasing signal frequency. The
      result is to restore the lost high-frequency response of the amplifier without also
      increasing the low-frequency response.
           T1, the transformer, plays a crucial part in the amplifier’s operation. As current
      is pulled through its primary coil, it generates a magnetic field that impinges on, or
      cuts across, its secondary coil, the one on the right. As the current in the primary gets
      stronger and weaker in step with the signal, the changing magnetic field generates
      a current in the secondary that makes its voltage rise and fall in step. That changing
                      Chapter 8      Roadmaps and Street Signs: Diagrams 157

voltage couples the new, amplified signal to the next stage. There are other ways to
couple a signal, without a transformer, but using one has advantages in some kinds
of circuits, especially those employing the transformer as a tuned circuit resonating at
a specific frequency. Radio receivers use lots of tuned amplifiers of this sort to pick
out the selected signal from all the others hitting the antenna. A capacitor across
the transformer, shown in Figure 8-4 by the optional C3, is a dead giveaway that an
amplifier is tuned. C3 could be on the other side of the transformer, too, and still have
the same effect in tuning it to a desired frequency.

Thar She Blows
Now that we’ve explored how the amplifier stage is supposed to work, let’s see what
the effects of malfunctioning parts would be. Again, starting at the top, how would a
bad transformer affect the performance of the circuit?
     If the transformer were open, no current would pass through its primary, so the
collector of the transistor would read 0 volts. Transformers in small-signal circuits
don’t pass much current, so an open winding is unlikely, but it can happen. By the
way, to estimate how much total current the winding might have to handle, just divide
the power supply voltage by the value of R3, a la Ohm’s Law. This pretends that the
transistor and the transformer’s winding have no resistance, so the real value will be a
bit less, but at least you’ll know the approximate upper limit.
     If the transformer had a short, its resistance would decrease and it’d be harder
for the transistor to pull its lower winding connection toward ground, so the voltage
at the transistor’s collector would be close to that of the power supply, with little
signal variation. Depending on the total current through the circuit (limited by R3, as
described above), the transistor might get hot or even be blown.
     If R1 were open, there’d be no bias current going to the transistor’s base, so the
transistor would be turned off. Its collector would be at the same voltage as the power
supply, and its emitter would be at ground potential, 0 volts. The signal itself might
have enough current to turn the transistor on a little bit during the positive half of
its waveform, resulting in a weak, distorted mess of negative-going signal excursions
appearing at its collector. (Remember, the amplifier inverts.)
     Shorted resistors are pretty much unheard of, but, for the sake of this thought
experiment, we’ll consider what would happen if one did short. If R1 shorted, the
base would be biased to the power supply’s full voltage. The transistor would be
fully turned on no matter what the incoming signal did. The collector would read
somewhere close to 0 volts, and the transistor might be hot, as might be R3.
     If R2 were open, the bias would be too high, with much the same result. The
signal’s influence might result in some output, though only on the negative-going half
of its waveform. Those negative incoming peaks would cause a rise at the output,
thanks to the inversion. As before, you’d get a weak, distorted mess, but the collector’s
DC level would be low, not high.
     If R2 were shorted, the base would be pulled down to 0 volts, resulting in the
same no-bias conditions you’d get if R1 were open, except that the signal could not
produce any output at all, because it would be shorted to ground as well.
158   How to Diagnose and Fix Everything Electronic

           If C1 were open, no signal would get to the transistor, but the DC voltages on Q1
      would look just fine.
           If C1 were shorted, the base bias would be influenced by the previous stage,
      resulting in unpredictable behavior. If the bias were pulled toward ground, Q1’s
      collector would rise, clipping off the top of the output signal if it got too high. If the
      bias went high because of voltage being fed in from the previous stage, the transistor
      would turn on too hard, the collector would be low, and the signal would be cut off
      toward the bottom.
           A shorted or leaky input coupling capacitor is something you might actually run
      into, particularly when it’s an electrolytic or tantalum cap. With a leaky one, it can
      be maddening to try to deduce why the stage behaves so oddly. If you disconnect the
      input cap and the DC voltages on the transistor change, the cap is letting some DC
      pass through it. It’s leaky.
           Now to the heart of the stage: the transistor. This is the component most likely to
      cause trouble. Transistors can fail in numerous ways. They can open from emitter to
      collector, usually as a result of overcurrent. They can also short that way, and often
      do. A short or an open can occur from base to emitter as well. I’ve seen transistors
      with all three leads shorted together like one big piece of wire!
           Transistors can be leaky, too, allowing current to pass when it should be cut off,
      or even to move backward through their junctions. A very small amount of reverse
      leakage is normal, actually, but it’s not enough to affect the circuit’s operation. A
      leaky transistor allows much more reverse current, and that’ll produce all kinds of
      unpredictable effects. The little monsters can also become thermal, changing their
      gain and leakage characteristics as they warm up.
           An open between any two junctions will result in no output. TP1 will be at the
      power supply voltage, because the transistor will not pass any current toward ground.
      Even if the collector-emitter junction is fine, an open base junction will prevent the
      transistor’s being turned on.
           A shorted collector-emitter junction, which is quite common, will appear as if
      the transistor were turned on all the time, all the way. The collector voltage will be at
      or very near zero. A shorted base-emitter junction, also common, will pull the base
      bias and incoming signal down toward ground through R3, and the transistor will not
      turn on. So, the collector voltage will not pull down and will be at the power supply
      voltage. A short from collector to base will turn the transistor on, having the opposite
      effect.
           A quick-and-dirty way to hunt for transistor shorts is to check the DC levels on
      all three leads. If any two are exactly the same, the part may very well be shorted. If
      they’re even a little bit different, a short is far less likely.
           If a transistor has leakage, it can act strangely. If collector current flows into the
      base, for instance, it can overbias the part. Depending on how much current leaks, the
      transistor may still work to some degree. If a stage acts wonky but everything seems
      to measure okay, and especially if the behavior changes with temperature, leakage is
      likely.
                             Chapter 8     Roadmaps and Street Signs: Diagrams 159

Switching Power Supply
Switching supplies range from moderately complex to ridiculously so. Rarely will you
find one that looks especially simple. If you think I’m kidding, crack open a modern
AC adapter. Even those inexpensive little wall warts are stuffed with chips, diodes,
transistors and regulators, along with a smallish transformer and the usual plethora of
resistors and capacitors.
     Switchers producing only one output voltage are less dense, with a straightforward
arrangement for regulating it. More elaborate circuits can have multiple output
voltages, overcurrent sensing, and other fail-safe protection measures adding to their
parts counts.
     At their core, though, switchers are not that complicated. Their basic operation
is pretty much the same, regardless of the frills. So, let’s strip away the doo-dads and
look at what makes these omnipresent beasts purr.
     Take a look at Figure 8-5 for a simplified schematic of the sort of power supply
you’re likely to encounter in many modern products, from LCD monitors to big-screen
TVs and audio gear. How do we know it’s a power supply? The presence of the AC
line input at the far left is a dead giveaway. Is it a linear supply or a switcher? Notice
that the AC line goes directly to a bridge rectifier and then to a transistor, before
reaching the transformer. That’s the classic switcher design: the AC gets changed to
DC, stored in a big electrolytic capacitor, and then chopped by a transistor which feeds
the transformer. So, this definitely is a switcher. In a linear supply, the AC would go
straight to the transformer; rectification, filtering and regulation would be done on the
other side.
     That big transistor below the transformer is the chopper. It switches on and off
at a high frequency, pulling current through the transformer to generate a pulse of
magnetism each time it turns on. The chopper is connected to the AC line, along with
everything else on that side of the transformer.
     The transistor is driven by pulses from the pulse-width modulator. That chip keeps
an eye on the supply’s output voltage and adjusts the duty cycle, or on-off ratio, of the
pulses it feeds to the transistor’s gate. The wider the pulses, the more energy will flow
across the transformer, and the more power will fill up the output capacitor, keeping
the voltage from sagging when the supply’s load increases. If the load decreases and
the output voltage starts to rise, the chip notices that and narrows the chopper’s pulses,


                                                              High-Speed
                                   +                           Rectifier +
            Fuse                             +                                                DC
     Hot                                                                            Output    Output
AC                                            Storage                               Voltage
                                             Capacitor                  Output      Monitor
  Neutral
                    Noise                                              Filtering
                                   –
                    Filter
                                Bridge                   Chopper   Pulse-width      Opto-
                               Rectifier                           Modulator       Isolator


 Figure 8-5        Switching power supply
160   How to Diagnose and Fix Everything Electronic

      bringing the output back down to its correct value. That regulation effect keeps the
      output voltage steady as the circuit being powered varies its demand for current.
           Between the output stage and the pulse-width modulator is an opto isolator,
      which is nothing more than an LED and a light-sensitive transistor in one case. It’s
      there to pass information about the output voltage back to the chip without forming
      an electrical connection between the two. Lack of a connection keeps the output side
      of the supply (everything to the right of the transformer) isolated from the AC line, and
      thus safe. Pretty simple story, isn’t it?

      Pop Goes the Switcher
      Let’s look at what would happen if the major components failed. Starting with input
      from the AC line, our first major component is the bridge rectifier. It could fail in
      several ways, but the most common problem is an open circuit in one of the four
      diodes. That’ll result in half of the AC waveform’s not getting transferred to the output
      of the bridge. With no capacitor to smooth things over, it would look like Figure 8-6.
           Since a capacitor is storing the charge, you’ll see a much lower-than-normal
      voltage at its positive terminal, along with a droop where the missing segment should
      be replenishing it. See Figure 8-7. The chopper may still run in this condition, but it
      probably won’t.
           One of the diodes could short, resulting in reverse-polarity voltage getting where
      it shouldn’t. In that case, expect the supply’s fuse to be blown.
           The most common failure in a switching supply is a bad chopper transistor.
      It operates at high voltages and takes a lot of stress. If it’s shorted, the fuse will be
      popped and the supply will be dead as a doornail. If the transistor is open, the supply
      will still appear dead, of course, but the fuse will be good and the big capacitor will
      have a full charge of a few hundred volts on it.
           If the PWM IC is dead, there will be no pulses at the base or gate of the chopper.
      The IC could appear bad, though, due to other factors. First, it needs some voltage to




       Figure 8-6     Missing rectified AC               Figure 8-7   Droop due to
       segment                                           missing segment
                      Chapter 8       Roadmaps and Street Signs: Diagrams 161

run, even before the chopper starts, so there could be a bad diode, zener or small cap
in its standby voltage supply. Also, if the output voltage of the entire switching supply
goes abnormally high, the chip will sense it and shut down. Typically, though, it will
try to restart every second or so, resulting in a chirping noise from the transformer.
Most switchers will also do that if the load they’re driving pulls too much current,
dragging the voltage down below what the supply can replenish.
      Some switching supplies employ a crowbar circuit intended to blow the fuse if the
output voltage goes too high. Crowbars usually employ a silicon-controlled rectifier, or
SCR. This component resembles a transistor, with three leads, but is really a rectifier
with a control gate. Its symbol looks like a diode with a third lead sticking into the
junction. As with a transistor, an SCR’s control gate turns it on, but that’s where
the similarity ends. Once tripped, the SCR stays on until the AC waveform changes
polarity, regardless of the control input. Also, the gate can only turn it on, not off.
      SCRs make great crowbars. Placed directly across the AC line, but just after the
fuse, the SCR normally doesn’t conduct, so it has no effect on the circuit. If the output
voltage goes abnormally high, a detector circuit trips and sends voltage to the SCR’s
control gate, turning it on. The short across the line blows the fuse and stops the
supply, protecting whatever gear it’s powering, along with the supply itself. If you
run into a switching supply with a blown fuse, but the chopper and bridge are good,
it’s reasonable to suspect that the crowbar tripped, and there’s some problem with
the voltage regulation system. Open zeners in the regulator circuits often cause this
condition.


Push-Pull Audio Amplifier
Let’s try another example. Figure 8-8 shows a slightly simplified channel of a typical
audio amplifier. The design has the somewhat humorous but also descriptive name
“push-pull” because it splits the incoming audio waveform into two halves, separately
processing the positive and negative portions of the signal. One half of the amplifier
pushes the speaker cone outward and the other half pulls it back in. It’s how most
modern audio amplifiers are built.
    The example shown in Figure 8-8 is a true bipolar circuit, powered by positive and
negative voltages with respect to ground, shown by +V and –V. The only components
connected to ground are the filter capacitors, C5 and C6, on the power supply rails,
and the speaker.
    Some similar amplifiers use only a single polarity. That works fine, but it means
that there will be a DC offset at the output of half the supply voltage, so that the signal
can swing equally up or down before hitting the limits of the rail or ground. Such a
unipolar design will have an output capacitor to block that DC offset from reaching
the speaker and keeping its cone pushed halfway out (not to mention wasting a lot of
power and heating up the speaker’s voice coil).
    Working from left to right, as usual, we see the input stage, which amplifies the
incoming signal enough to drive the next stage, consisting of Q2 and Q3. The opposite
polarities (NPN and PNP) of those transistors mean that opposite halves of the
signal will turn them on. The signal is coupled to them by C3, which blocks any DC
162   How to Diagnose and Fix Everything Electronic

                                                                                   +V
                                                                         R8         +
                                                                                     C5
                                                                    Q4
                                                    R5         Q2
                               R3
                                     C3
                         R1                              +                              8 Ω Speaker
                    C1                         D1         C4
                                    Q1
            Input                                   R6         Q3
                         R2
                                                                     Q5
                                          C2
                               R4
                                                    R7

                                                                              R9
                                                                                   –V
                                                                                       C6
                                                                                   +


             Figure 8-8       Push-pull audio amplifier


      component from Q1 from influencing their biasing. The bias network of R5, D1, R6
      and R7 keeps the transistors turned on just slightly, so there’s no dead spot to cause
      crossover distortion when the input signal’s waveform is less than ± 0.6 volts or so. C4
      couples the signal to both transistors. Their outputs drive Q4 and Q5, which provide
      enough current gain to move a speaker cone. In a real amplifier, a little bit of the
      output would be fed back through a few resistors and capacitors to the input stage in
      a negative feedback loop, to correct for distortion introduced by the imperfect nature
      of the amplifying elements (transistors) by reintroducing the same distortion upside
      down, cancelling it out. We’re omitting those parts here to keep things clear.

      Sounds Like a Problem
      Let’s look at how malfunctions in each stage would affect the amplifier’s behavior. If
      Q1 or its surrounding components broke down, no signal (or perhaps a very distorted
      signal) would emerge from C3. Because C3 blocks DC, the badly skewed voltages at
      the input stage would not affect the rest of the circuit, so further transistors would not
      be damaged by being turned on too hard and pulling too much current.
          In a direct-coupled circuit, though, the input stage would be carefully designed to
      have no significant DC offset, and there’d be no C3. Should the input stage of a direct-
      coupled amplifier malfunction and generate a lot of DC offset, as may happen with
      a bad transistor, that offset could wreak havoc on the rest of the transistors, possibly
      blowing all of them.
          If D1 opened, the top half of the amp would be turned on very hard by the bias
      provided by R5. Q2’s base would go very positive, turning it all the way on. That would
      pull the base of Q4, the PNP transistor, down toward ground through the speaker,
                      Chapter 8       Roadmaps and Street Signs: Diagrams 163

turning that transistor all the way on as well. Both transistors would get quite hot and
might be destroyed.
     Meanwhile, R7 would pull the base of Q3 very negative, which would turn that
PNP transistor all the way on as well. That would turn on Q5, too, and funerals for Q3
and Q5 would likely be in order.
     Normally, a push-pull amplifier has one half on at a time, with the other half
conducting only slightly until the signal’s polarity flips, reversing the process. Power
supply current passes through the output transistors, one at a time, to the speaker
and then to ground. In this case, both halves would be turned on at the same time,
effectively shorting the +V and –V lines through the output transistors. Yikes! You can
imagine the results. Smoke, burned emitter resistors (R8 and R9), blown transistors,
an unholy mess…all from one bad diode.
     If D1 shorted, though, the results would be different. The bias would become
unstable and the amp’s DC offset would swing around with the signal and distort it
badly, but the transistors would probably survive because the bias wouldn’t be so far
out of whack that it’d turn them all the way on.
     If C4 opened, the top half of the amp would work but the bottom would get no
signal, so severe distortion would occur, with only one-half of the waveform present
at the output. If C4 shorted or got leaky, the result would be similar to what you’d see
if D1 shorted: the bias would get wonky, the amp would distort, but parts probably
wouldn’t be damaged.
     An open in Q2 or Q3 would turn off the corresponding half of the amplifier, with
loss of one half of the signal waveform. A short in one of those transistors would be a
much more serious matter.
     Q2 and Q3 are referred to as driver transistors, because they drive the output
transistors, Q4 and Q5. At this point, the circuit is direct-coupled, so a short in a driver
will turn its output transistor fully on, probably blowing it. At the very least, there will
be a lot of DC at the speaker terminal, and the speaker may also be blown from all the
current passing through it.


Mega Maps
Highly complex schematics can be tough to follow, with all kinds of confusing signal
and power lines running every which way. Especially with such schematics, a block
diagram can be incredibly helpful. Devices with multiple boards may have many
connectors and cables shuttling signals back and forth. The connectors are valuable
focal points. If you’re not sure what goes where, trace the schematic back from a
connector to see if you’re in the right place. Still not sure? Grab the block diagram and
see if that area goes where you think it does.
    Another great place to find a signpost on a big schematic is an input or output
point. Jack and speaker symbols stand out because there are so few of them on any
given diagram. Trace the lines back to find what’s feeding them.
    Transformers also jump off the page. Whichever symbol you choose, you can train
yourself to scan a schematic and find it, disregarding the others. It’s a lot harder to do
that with resistors and capacitors, of course, because there are so many of them.
164   How to Diagnose and Fix Everything Electronic


Give It a Try
      If you have any schematics, now’s a good time to pull them out and practice your
      reading. If you don’t have any, go on the Internet and search for some. Radios, TVs,
      and CD and DVD players all make for good reading material. See what you can
      identify. Try to find the following stages and sections.


      Radios
      Look for the front end, which is the first section accepting input from the antenna. In
      an analog radio, you’ll see tuned circuits with variable capacitors. See if you can follow
      the input signal up to the mixer, which is where it gets mixed with the local oscillator.
           Digital sets don’t have variable caps. Instead, look for a big chip and a bunch
      of surrounding parts forming a frequency synthesizer. How do you find that? It’ll be
      connected to an LCD and probably to a keypad, too.
           Once you’ve found these stages, follow the signal through the mixer. The mixer
      may be a chip, or it may be four diodes in a ring configuration that looks a bit like a
      bridge rectifier, except that the diodes are facing different ways. Some mixers use a
      dual-gate MOSFET transistor, with the incoming signal fed to one gate and the local
      oscillator fed to the other. The mixer’s output goes to the IF, or intermediate frequency,
      stages. Look for transformer coupling between stages. Most modern radios, analog or
      digital, convert the frequency twice (and sometimes even more), so you should see a
      fixed-frequency oscillator feeding another mixer stage, followed by more IF stages.
           From the IF section, the signal gets demodulated, or detected. This is the process
      of extracting the information—audio, video or data—that was originally impressed
      on the signal at the transmitter. AM detection may involve nothing more than a
      diode and a capacitor. FM is a bit more complex, and data can involve all sorts of
      decoding circuitry. Data detection used to require a lot of chips, but these days a small
      microprocessor or a DSP (digital signal processor) chip may do the work. DSPs get
      used for enhancing voice signals, too, especially in modern communications receivers
      and transceivers.


      CD and DVD Players
      Despite the low prices of disc players, getting the data off an optical disc is not a
      simple task. It involves three servo systems working together to find the microscopic
      tracks, follow them as they pass by, and keep constant the rate at which their data is
      read out. The laser head must properly track the absurdly tiny groove, even though
      normal eccentricities in the geometry of the discs are many, many times the size
      of the grooves themselves. This is a three-dimensional problem, with the wobbling
      distance between the head’s lens and the disc surface requiring a dynamic focusing
      servo to keep the beam size at the point it meets the track small enough to grab just
      one bit of data at a time.
                       Chapter 8       Roadmaps and Street Signs: Diagrams 165

     Unlike analog records, CDs and DVDs use constant linear velocity (CLV) to pack the
data in with maximum space efficiency. With CLV, the speed at which the laser head
sees the track go by is constant, regardless of whether the distance around the track
is short, as it is near the center of the disc, or long, as it is near the outer edge. So, the
disc must spin faster at the start and gradually slow down as the disc plays, reaching
minimum rotational speed as the head plays the longest tracks, nearest the outside.
Accomplishing this automatic speed control requires yet another servo to keep the
disc spinning at exactly the required rate.
     Take a look at a disc player’s schematic. Can you find the laser head? Notice that
it has more than one photodetector (light-sensitive transistor). Three are used to keep
the laser beam centered on the track. Look also for the servo coils used to float the
lens and make it dance in step with disc wobbles.
     Trace back from the head and see if you can locate the head preamp, which boosts
the weak signals from the photodetectors. It should be a chip of medium density and
will have a few test points at or very near some of its pins.
     Trace back from the servo coils and see if you can find the focus servo section and
the tracking servo, too.
     Find the sled motor, which moves the head across the disc as it plays, and the
circuit driving it. Most motor driver circuits have transistors between the chips
controlling the motion and the motor because the chips can’t supply enough current
to run the motor directly. Since the sled motor has to run in either direction, look
for what’s called an “H bridge” configuration of the driving transistors, in which the
connection to them is in the middle, with each wire going to the motor coming from
where two transistors meet. It looks like the letter H, hence the name. Neither wire
goes to ground, so the controlling circuitry can flip the polarity to the motor at will,
reversing its direction. Some H bridges are implemented on a single chip, but many
are still made from separate transistors. See Figure 8-9.
     Now search for the disc motor, which spins the disc. Its driver circuitry will look
similar to the sled motor’s circuit, but not identical. It’ll probably have a transistor as
well, but it has to spin only one way, so no H bridge is required.

                     IN                     Vcc                    IN




                                           Motor




                      Figure 8-9      Typical H bridge
166   How to Diagnose and Fix Everything Electronic

      Keep Reading
      You’ll find lots of schematics on the Internet. Get a few and practice reading them,
      focusing on signal flow and organization. Try to deduce which components are
      generating or passing signals, and which are support systems for the central players.
      Look for coupling components, filter and bypass capacitors, power supply sections,
      digital control systems, voltage regulators, oscillators, and so on. After awhile, reading
      a schematic will be as familiar as reading a book. You’ll be able to take one look and
      recognize the sections and stages.



But I Ain’t Got One!
      As important and useful as a schematic is, you will find yourself working on many
      devices without one, simply because you can’t get it or it costs more than you want to
      pay. Without the roadmap, how do you find your way around?
           It’s a lot tougher, but if you keep the overall circuit functions in mind, you can
      find the major sections and determine whether they’re working properly. When
      you get to a suspicious stage, try drawing your own mini-schematic by tracing the
      connections of the active element and its surrounding components on the board.
      Sometimes seeing it in front of you will illuminate the concept of the design and lead
      you to good troubleshooting ideas.
           Once you locate the malfunctioning section, your understanding of stages and
      signal flow will help lead you to the problem. At least, that’s the way we want things
      to go. Sometimes it does, sometimes it doesn’t. In very complex devices, it’s easy to
      get lost and go around in circles when you’re flying blind, without a diagram. Luckily,
      most malfunctions are due to power supply problems, bad connections and faulty
      output stages, which are relatively easy to track down, even without a schematic.
      When complex signal processing stages don’t work right, you may not be able to
      determine why, especially in modern devices with hundreds of parts the size of grains
      of salt. Even in those situations, though, you can learn a lot by tracing back from
      input and output jacks and poking a scope probe on what look like input and output
      points of successive stages. You just might find the spot where the signal disappears
      and zero in on the bad component.



Your Wish Is Not My Command
      Let’s take a case and work through it without a diagram. This will be a nice little LCD
      TV that refuses to respond to its remote control. It functions fine with the front-panel
      buttons, but the remote does nothing.
           Is the remote working? How can we tell? If only we could see infrared light! Got
      a camcorder or a digital camera? Those can see infrared. Even though they have
      filters to block it, some IR light gets through. Point the remote at the lens and hit
      one of the buttons while looking at the camera’s display screen. If you see a flashing
                       Chapter 8      Roadmaps and Street Signs: Diagrams 167

light, the remote is working. Could it work but not be sending the right codes? Yes,
theoretically, but I’ve never seen it happen, except in the case of a universal remote
set up to operate the wrong device. That’s user error, not a repair problem. If the
remote’s IR LED lights up when you press a button and stops when you let go, you
can assume it is working properly.
     In this case, the remote works. So, why can’t the TV see it? Something in its remote
receiver circuitry is out, and we’re going to hunt that problem down. Naturally, this
newer product has no available schematic, so we’re on our own.
     The first thing to find is the photodetector that picks up the remote’s signal.
Most products use a prefab remote receiver module containing a photodetector and a
preamp. The module is usually in a little metal box (see Figure 8-10), though in small,
battery-op gadgets it’s more likely to be a bare plastic part. It sits just behind the front
panel of the unit so the photodetector can see out through the panel’s plastic bezel.
Yup, there it is. It’s mounted to the circuit board and connected by three lines. Let’s
stick our scope probe on them and see what’s there.
     First things first: we have to connect the scope’s ground lead to circuit ground
and then power up the TV. To begin, let’s set the scope for DC coupling, 1 volt per
div, and put the trace at the bottom of the screen, since we expect positive voltages
in this negative ground TV. (We did verify that the negative power supply line went
to circuit ground, right? Of course we did.) Hmm…one of the module’s three lines
seems to have nothing on it at all. Why would a connection have no signal? Could
finding the fault be this easy? Nah! The line goes to a big area of copper on the board.
It’s ground! Okay, one down. The second line shows a steady voltage that doesn’t vary
when the remote is pointed at it and a button is pressed. That one must be the power


                                         IR Module




                  Figure 8-10      Remote control receiver module
168   How to Diagnose and Fix Everything Electronic

      supply voltage running the module. The third line has what looks like a little noise on
      it. When a remote button is pushed, that line becomes an irregular pattern of pulses
      at just under 5 volts peak to peak. Aha! That’s the code the remote is flashing. We’ve
      found the module’s output line.
            The good news: we’ve proved the receiver module is okay. If it weren’t, the output
      line wouldn’t do a darned thing when the remote sent its optical signal. It’s showing
      the transmitted code, so it’s working. The bad news: we still have no idea why the TV
      won’t respond to it.
            So, we follow the output line and see where it goes. It appears to terminate at a
      diode that feeds a transistor. The diode’s purpose isn’t obvious, but there it is, so we’d
      better check it. We scope it to see if the signal from the module is getting there. It
      is. At the other end of the diode, the signal is smaller but it’s still there. Because the
      signal is smaller, we can figure that the diode is not shorted. And, because it’s there at
      all, the diode is not open. It could be leaky, but most likely it’s fine. Time to focus on
      the transistor.
            One of the transistor’s three leads, presumably the base or gate, is connected
      to the diode. Sure enough, the remote’s signal is there. The other two leads are at 0
      volts, though! No DC, and neither shows any activity when the remote is activated.
      No wonder the TV never sees the remote’s signal. It’s a dead end. Blown transistor! At
      least, that’s what a novice would think. In truth, we don’t have enough information to
      draw that conclusion yet, so we look further.
            One lead goes to circuit ground, so we wouldn’t expect anything there. The other
      should have some voltage, though, right? Tracing from that one, we come to a couple
      of resistors, and it isn’t clear where they go. Since the transistor needs power to
      function, one of those resistors must go to a power supply feed point. And since the
      transistor’s output signal has to feed some other part of the circuit that knows what to
      do with it, the other resistor must be coupling it to another stage. But which one does
      what?
            Putting the scope probe on the opposite end of each resistor, we find that neither
      has anything on it there either. To use the technical term, deadus doornailus. If the
      transistor were shorted, that would pull its output line to ground, but not the other
      end of the resistor feeding power to it. That’s the whole point of having a resistor
      there: to limit the current and avoid pulling the whole supply down when the
      transistor turns on and connects the line to ground. So, the resistor’s far end, fed from
      the power supply, should still have voltage on it no matter what the transistor does.
      But the voltage isn’t there. This suggests the problem lies elsewhere, and the transistor
      is probably not the culprit.
            Tracing each resistor, we see that one goes directly to a huge chip. The other goes
      someplace far away we can’t easily find. The one going to the chip is most likely the
      transistor’s output, feeding the remote’s signal to the microprocessor for decoding.
      The other one, then, has to lead to the missing voltage. So, we follow it as it snakes
      along. Eventually we find it leading to a little jumper wire, the other end of which
      connects to a fairly large land with lots of other parts and traces going to the same
      place. That should be the power supply feed point. Poke ye olde probe, and there’s
      five lovely volts. Yee hah, it works! Oh, wait a minute, why isn’t it getting to the
      resistor?
                      Chapter 8      Roadmaps and Street Signs: Diagrams 169

     We turn the set off and watch the voltage on the feed point die away to zero. Out
comes the DMM, set to the ohms scale. That’s odd; there seems to be no connection
from the feed point to the resistor. A check of our tracing confirms we haven’t made a
mistake. Nope, we’re in the right place. We flip the board and take a good look at that
jumper wire’s solder joints. One of them looks cracked! We touch it up with the iron
and recheck the line for continuity. Now there’s a connection! You can guess the rest.
We power up the set and it works, remote and all!
     That is a very typical repair case. And, as you’ve seen, it can be solved without a
diagram, by understanding basic circuit function and applying a little logic. Had there
been voltage at the power supply side of the resistor but not at the transistor it was
feeding, yanking the transistor and checking it for shorts would have been the next
step, and it’s a pretty safe bet the part would indeed have been shorted from collector
to emitter, pulling the applied voltage to ground. The lack of voltage at the resistor’s
far end was the critical clue in acquitting the transistor, tracking down the real culprit
and solving the case. Verdict: time served, and free to go!
This page intentionally left blank
Chapter                 9
 Entering Without Breaking:
 Getting Inside

 O     nce upon a time, getting at the circuitry of a consumer product was a piece of
       cake. Remove a few screws, pop off the back, and there it was. Providing access
 to the inner workings was a tradition begun in the vacuum tube days, when the unit’s
 owner needed to get inside on a frequent basis to change tubes, lamps and fuses.
 After semiconductor technology replaced the troublesome tubes with considerably
 more reliable transistors, there was still the expectation that the buyer might have
 a legitimate need to reach the circuit board. Early solid-state products even put the
 transistors in sockets! As semiconductors got sturdier, the sockets went away, which
 was good since their flaky connections caused more failures than did the transistors
 themselves. There were still lamps and fuses to contend with, though, and access was
 expected and easy.
      That was before today’s age of complex, ultra-miniaturized circuits, complete
 lack of user-serviceable parts (mostly thanks to the LEDs that replaced lamps), and
 lawsuits. These days, no manufacturer wants you anywhere near the stuff under
 the hood.
      Consequently, many equipment cases are deliberately sealed, or at least made
 pretty tough to open. Most AC adapters are ultrasonically welded together and have
 to be cracked open; even the expensive ones are considered non-repairable items by
 their makers. Lots of laptop computers, video projectors, MP3 players and TVs sport
 hidden snaps, so they won’t pop apart even after you remove the screws. See Figure
 9-1. And anyone who’s ever tried to take apart a certain American computer company’s
 sleekly designed products knows the meaning of frustration; they’re clamped together
 seamlessly and tightly in a clearly deliberate attempt to keep you out.
      So how do you open up these crazy things? Sometimes, others have suffered
 before you and have posted step-by-step disassembly instructions on their Web sites,
 with clear photos of the whole process. If you can’t fathom how to get something
 apart, it pays to do a Web search on “disassemble ______,” with the blank filled in by
 the name or model number of your product. Some companies selling parts post these


                                                                                  171
172   How to Diagnose and Fix Everything Electronic




                 Figure 9-1     Hidden plastic snap



      helpful tutorials, facilitating your getting to the point of having a reason to order
      their wares.
          If you can’t find disassembly instructions, you’ll have to wing it. With or without
      help, this is the stage at which you have the most opportunity to wreck your device!
      Without X-ray vision, you can’t know if that tiny screwdriver you’re using to pry the
      case halves apart or unhook the hidden snaps you’re not even sure are there might be
      ripping a tiny component off the circuit board or causing some other drastic damage.
      You also have no idea whether a ribbon cable may join the two halves and be torn
      when they suddenly separate. And nothing is more frustrating than thinking you’re
      about to repair something and destroying it instead, before you ever get a chance to
      look for the original problem.
          Despite all this gloom and doom, you can get into nearly any product successfully
      and safely if you’re careful and take your time. Disassembly is not a trivial process;
      expect it to take a significant portion of the total repair time, at least with the smallest,
      most complex devices. There are some tricks to the endeavor, and we’re going to
      explore them now. But first, some rules:

       •	 Rule number one Always disconnect power before taking something apart.
          This is true with battery-operated products as well as AC-powered ones. Even if
          there’s no danger to you, you have a much higher chance of damaging the device
          if power is connected when things come apart, whether the unit is turned on or
          not. Pull the plug, yank the batteries.
                   Chapter 9      Entering Without Breaking: Getting Inside 173

    •	 Rule number two Remove everything you can before going for the screws.
       Battery covers, recording media (tapes, discs, memory cards), rubber covers over
       jacks, lanyards—if it comes off, take it off. There’s no need to pull knobs from a
       front panel if you don’t anticipate having to remove the panel, but that’s about it.
       Everything else should go. And don’t be too surprised if you later wind up having
       to take off those knobs after all.
    •	 Rule number three Never force anything. If the case won’t come apart, or
       some corner seems stuck, there’s a reason. Perhaps snaps are hiding on the inside
       of the plastic. Screws are sometimes hidden under labels and rubber feet. Run
       your fingernail over labels, looking for the indentation where a screw head might
       be lurking. If you find one, peel back the label just enough to get to the hole. Peel
       back the rubber foot near the stubborn area; just because the other feet aren’t
       hiding screws doesn’t mean this one isn’t. You’ll probably have to glue the foot
       back on later, but it’s a necessary consequence of peeling the original adhesive.
    •	 Rule number four Don’t let frustration drive you to make a destructive
       mistake. Even the calmest tech can get riled up when a recalcitrant patient tries
       his or her patience badly enough. The most common errors are to start moving
       too fast, to force something, or, in extreme cases, to smack the casing, hoping it’ll
       loosen up. Bad idea! Okay, I admit it: I once threw a really nice, rather expensive
       pocket stereo cassette player against the wall after a maddening, futile hour
       of trying to take it apart, but I don’t recommend the exercise. It felt good, but,
       needless to say, that repair job was over before it began. Plus, for weeks I was
       stepping on pieces of that poor little thing I’d murdered. I couldn’t help but think
       of Poe’s The Tell-Tale Heart every time something went crunch underfoot. At least
       it was my own player, and not something I’d have to replace for a customer or
       lose a job over.



Separating Snaps
   Popping apart hidden snaps is almost an art form in itself. First, be absolutely sure
   you’ve removed all the necessary screws. Take a look at the bottom of the unit to see if
   there might be slots into which you can put a screwdriver to pop the snaps. Those are
   common on AC-operated devices whose bottoms face shelving, but not on pocket toys.
   If you do find slots, shine a flashlight into one and see if you can deduce what needs
   to be pressed in which direction to unhook the snap. Pop one open while pulling the
   case halves apart with your free hand. To prevent accidental reinsertion, keep holding
   them apart while you do the next snap on that side of the case. Once you have a
   couple of snaps open, you won’t need to continue the forced separation, and the rest
   should open easily.
        If you find no slots, pick one side of the case and press on the seam around
   the edge, looking for inward bending of the plastic. Move slowly and feel for slight
   movements indicating where hidden snaps may lie. When the plastic gives a little,
   press harder and attempt to pull the seam up. If it won’t budge despite your best
   efforts, move to another part of the case and try again. After you get one snap undone,
   the rest will release a lot easier.
174   How to Diagnose and Fix Everything Electronic

          If no amount of effort will release the snaps, you might be tempted to slip a small
      screwdriver into the seam and pry it open. It’s a last-ditch procedure, but it usually
      works. It’s almost certain to break snaps, though, and cause some visible damage on
      the outside of the case.
          Even with the best technique, you’ll break a few while you get the hang of it, and
      occasionally even after you’re an expert. Luckily, it’s not the end of the world. Often
      the loss of a snap or two has little or no effect on a product’s integrity, but sometimes
      the reassembled case can feel loose or have a gap along the desnappified seam. Just
      save the broken pieces in case you need to melt them back on later.



Removing Ribbons
      Ribbon cables have replaced wires in small products. They offer much higher density
      with a lot less mess, and we couldn’t have today’s complex pocket devices without
      them. The ribbons are delicate, though, and removing them from their sockets
      requires care.
           Some ribbon connectors have latches that press the ribbon’s bare conductor
      fingers against the socket’s pins. Others have no latches and rely on the thickness of
      the ribbon to make a firm connection. Either style may have a stiff reinforcement tab
      at the end of the ribbon, but the latchless style always does.
           Before pulling out a ribbon, take a Sharpie marker and put a mark on both the
      ribbon and the socket so you’ll know how to orient the cable during reassembly. Use
      a Sharpie; other markers may rub off. If other nearby ribbons are similar enough that
      you could possibly confuse which goes where, use a unique mark on each one.
           Now examine the socket closely. If you see small tabs at either end, it has a latch.
      Even without tabs, it might have a flip-up latch. Pulling a ribbon from a latch-type
      connector without opening the latch first can easily tear the ribbon. You don’t want
      that! Ribbon cables are custom made for each product, and you aren’t going to find a
      replacement unless you can dig up a dead unit for parts. A torn ribbon usually means
      a ruined device.
           If there is no latch, grasp the ribbon at the reinforced tab and pull steadily. Don’t
      jerk or you’ll almost certainly destroy it. Pull gently at first and then harder if the
      ribbon doesn’t move. With some of the larger cables, you might be surprised at how
      much force it takes to get them out of their sockets. If you need to pull hard, hold the
      socket down on the board with your other hand to prevent ripping it from its solder
      joints. It’s rare for a ribbon to need that kind of force, but I’ve seen it a few times.
      Whatever you do, don’t pull on the unreinforced part of the ribbon; it won’t withstand
      the stress.
           If there’s a latch, open it first. There are two basic styles: slide and flip-up. Slide
      latches have little tabs at the ends of the socket. A fingernail or the end of a flat-blade
      screwdriver will pull them open. It’s best to open them at the same time, to keep the
      sliding part from getting crooked or breaking off, but gently opening them one at a
      time usually works fine.
                    Chapter 9      Entering Without Breaking: Getting Inside 175

       Flip-up latches open easily with a fingernail. You can use a screwdriver, but
   be careful not to scrape and damage the ribbon cable. Especially with a very small
   connector, open the flip-up slowly and carefully, as they break quite easily. It helps to
   pull them from near their ends, lifting both ends at the same time, rather than from
   the middle. If you break one, keeping enough pressure on the ribbon to make a proper
   connection with the socket is next to impossible.



Pulling Wire Connectors
   Larger items may have a mixture of ribbon cables and good old wire assemblies.
   Circuit board-mounted connectors for wiring rarely have latches. If they do, the latch
   will be large and obvious, something you squeeze with your thumb while pulling
   on the connector. Most wire connectors simply pull straight up. Reorientation is not
   an issue with these, but marking is still advisable if other nearby connectors could
   cause confusion. We’ve all been taught through the years never to pull on the wires
   when removing plugs, but that’s what you have to do with these because the plug fits
   entirely into the socket, leaving nothing else to grab. Grasp the wires, pull steadily
   without jerking, and the connector should pop out.
        Before you do, though, be certain there actually is a connector! Groups of wires
   sometimes terminate in what look like connectors, but the wires go right through the
   plastic and are soldered directly to the board. Obviously, you don’t want to pull on
   those.



Layers and Photos
   Remember those little cups I suggested you collect, way back in Chapter 2? Here’s
   where you will use them. Unlike the simpler products of yesteryear, modern gadgetry
   is often built in layers. Perhaps the topmost layer is a display. Under that lies a metal
   shield. Beneath that is a circuit board, and there’s another board under that one as
   well. Behind all of it is the battery compartment and a little board for connectors.
   Ribbon connectors join the layers, with several on each side of the main boards, and
   you can’t get to the lower layers without removing the upper ones first. Sounds like a
   huge product, doesn’t it? Perhaps a home theater receiver or a laptop computer? Hah!
   I just described a typical digital camera! Just wait till you try to get to the back of the
   mechanism in a pocket-sized MiniDV camcorder.
        To reach the innermost spaces, the layers have to be stripped away in precisely
   the reverse order of their original assembly. And, naturally, the problem you’re
   chasing is at the bottom layer, right?
        Each layer is held by screws, and they’re probably different sizes from those
   holding the next layer down. Some may even be different from others in the same
   layer. Not all screws always have to be removed; some only grip small internal parts,
   and unscrewing them may drop a nut or a washer deep into the works. Especially in
176   How to Diagnose and Fix Everything Electronic




                        Figure 9-2     Arrow indicating screw removal


      devices with motors or speakers, both of which use strong magnets, a lost metal part
      can get pulled in and cause real trouble later. Some companies stamp a little arrow
      by the screws that must come out, especially on the outside of the product’s case, but
      that’s not an industry standard. See Figure 9-2. If you do see the arrows, remove only
      the screws that have them. If there are no arrows, start with the screws near corners,
      and see if that frees up the case. No? Then you’ll have to remove them all and hope
      for the best.
            As you unscrew a screw, observe how it comes out. It should rise, indicating that
      it’s screwed into a fixed object, not a nut. If it seems very loose as soon as you start
      to turn it counterclockwise, screw it back in again and see if it tightens down without
      slippage. If not, then there probably is a nut on the other side, and you don’t want to
      unscrew it! Nuts are almost never used on the screws holding case halves together.
      How would the manufacturer tighten such a screw without having access to the nut?
      Once in a great while, you do find a nut or a little metal bracket on the inside, glued
      into a plastic shelf so it’ll stay put after the case is assembled. It’s rare, though, and
      found mostly on older gear.
            When you remove screws, take a good look at each one after it’s out. Pay careful
      attention to the length, comparing it to the last one. Very often, otherwise identical-
      looking screws are of different lengths, and putting one that’s too long in the wrong
      hole when you reassemble the device can make it poke into something, causing a
      short or other serious damage.
            As you take out screws from a layer, put them in one of those cups. If different
      lengths are used, make a quick drawing of which went where on a little piece of scrap
      paper or a sticky note, and put that in the cup too. Now put another cup into the first
      one, covering the contents of the lower cup. When you start on the next layer, put
      its screws into the open cup, and so on. If the device is especially complicated or has
      many layers, take a photo of each layer, with the stack of cups visible in the picture.
      That way, you’ll know which set of screws goes with which layer—just count the
      number of cups in the photo. You might be surprised at how easy it is to lose track
      of that after the unit has sat in pieces on your bench for days or weeks. When the
                    Chapter 9       Entering Without Breaking: Getting Inside 177

   disassembly operation is complete, you’ll have a stack of cups, with the screws from
   the last layer in the top cup. Place an empty cup in the top one to keep the screws
   securely inside, and put the stack somewhere safe and out of your way.



Opening a Shut Case
   Let’s take a look at some case opening procedures for common products, starting with
   the easiest and working up to the really challenging adventures.


   Receivers and Amplifiers
   Most shelf-style audio gear opens up with no hassle. You’ll find four screws, two on
   each side, two or three smaller ones at the top edge of the back panel, and perhaps
   one to three on top, just rear of the front panel. Unscrew them all, put them in a cup,
   and the top should slide off. Often, you’ll have to spread the sides slightly while lifting
   the back edge, as the front edge is under the top of the front panel.


   VCRs, CD and DVD Players
   Most VCRs open the same way, but there are some variations. Some have screws on
   the bottom instead of the sides. You won’t find one on top behind the front panel, but
   you may find some on the back’s upper edge. On many VCRs, you have to slide the
   top straight back before trying to lift the rear edge.
        CD and DVD players require much the same thing, but the front edge may have a
   lip fitting into a groove on the front panel.


   TVs and LCD Monitors
   Today’s flat-panel TVs and monitors usually unscrew from the back. The panels are
   recessed from the bezel around them, so you should be able to lay the set face down
   gently on your bench, after sweeping the table’s surface and checking for anything
   that could stick up and put pressure on the LCD or plasma screen. You may find lots
   of screws of various lengths. Be very careful to note which go where, because under
   those screws is the back of the display panel! You really don’t want to mix up the
   lengths and screw anything into that when you put it back together.
        Older CRT-type TVs have screws at the top and bottom of the back. They may
   be recessed, requiring a long screwdriver. It’s best to lay small sets face down on a
   pillow before removing the screws. Some larger units let you remove the back without
   upsetting the stability of the TV, but many require that you tilt them forward a little bit
   to get the back off. That can be disastrous if the set flops onto its face. If you can put the
   TV on the floor and tilt it against a wall only a few degrees, that’s your safest option.
        Just remember that the neck of the picture tube, which will be in your face when
   you pull off the back, is dangerous, both electrically and mechanically. Be extra-careful
178   How to Diagnose and Fix Everything Electronic

      not to hit the tube’s neck, and don’t touch the circuit board at the back. Nasty voltages
      live there, and they can persist even after the set has been turned off and unplugged.


      Turntables
      Turntables are an old technology, but they enjoy a following among audiophiles,
      so there are plenty of them still around. Also, turntables are uniquely shaped and
      somewhat delicate, making them awkward to service.
           A turntable’s platter may be driven by a rubber wheel, a belt or a direct-drive
      motor turning at the same rate as the record. Most better turntables are belt- or direct-
      driven. To change the belt, first lock down the arm so it won’t flop around and damage
      the stylus. Then lift the rubber mat on the platter and you’ll see the motor spindle
      somewhere on the left side. Putting the new belt on requires lifting the platter straight
      up and out. Most come off without removing anything, but some have a retaining clip
      around the spindle.
           For any other repairs, you’ll need to get to the underside of the turntable, which
      involves laying it on its face. To do that, put it on a pillow arranged such that the
      weight of the unit won’t be on the arm. Never put pressure on the arm assembly; the
      arm probably won’t survive.
           Before you flip the unit over, it’s wise to take off the stylus and put it aside, as it’s
      the most fragile, easily damaged element. Whack anything into it and it’ll get trashed.
      The quickest and safest way to get the stylus out of harm’s way is to pull the entire
      cartridge. Many later turntables use a “p-mount” cartridge that unplugs easily, with
      no individual wires and connectors to deal with. Some with the old-style mount have
      removable head shells. If you see a sleeve where the head meets the arm, it’s probably
      a removable shell. Unscrew the sleeve and it should pop right off.
           Once the stylus and/or cartridge have been removed, secure the arm with its
      retaining clip. Take a look at the back of the arm. If you see a little anti-skate weight
      hanging down on a wire, make a note of its setting and then remove it so it won’t get
      damaged when you flip the turntable over. If the primary counterweight slides on and
      off, slide it off after noting its setting as well.
           Remove the mat. Unless the platter is held on with a retaining clip, remove the
      platter too, so it doesn’t fall off.
           Many turntables are mounted on springs, so you need to hold the corners of the
      chassis as you turn it over or the machine can fall out of its base. Hold those corners,
      turn over the unit slowly, and place the turntable face down on the pillow, making
      sure none of the weight is on the arm. If there’s a bottom plate, remove the screws
      securing it, and it should come off.


      Video Projectors
      Be sure the lamp is completely cooled down, and take it out first! The bulb represents
      most of the cost of the projector. Plus, it’s fragile and contains mercury. Put the lamp
                Chapter 9      Entering Without Breaking: Getting Inside 179

assembly aside, far enough from the work that you won’t drop a tool on it or knock it
off the bench.
     Most video projectors have screws on the bottom. After their removal, the top half
will lift off. Some projectors are entirely snapped together, with no screws. Even with
screws, there may be hidden plastic snaps.
     On some units, the lens has plastic rings for focus and zoom that must be pulled
off before the case can be separated. Pry the rings off with your fingers, avoiding the
use of tools. If you must use a screwdriver, do so especially carefully.
     There may be ribbon cables between halves to connect the control buttons and
indicator lights. Separate the halves slowly to avoid tearing them.
     As you remove the case from a DLP projector, keep your fingers away from the
front of the unit, because the color wheel is just inside, and it’s fragile. Putting any
pressure on it is likely to result in its destruction. LCD units have no color wheels to
worry about.


Portable DVD Players with LCD Screens
These usually have screws of varying lengths in the back. After you remove those, the
back should come off, but make sure to have the unit lying on its face, because the
laser sled assembly can fall out and tear its ribbon cables if you hold it in any other
orientation. The assembly sits on rubber bumpers, and the back holds it in place. It’s
supposed to be loose, for vibration damping and skip resistance. See Figure 9-3.




           Figure 9-3    Inside a portable DVD player
180   How to Diagnose and Fix Everything Electronic

           If you need to get to the LCD monitor and its associated circuitry, including the
      backlight inverter, its screws are probably under the rubber bumpers on each side at
      the top. Check the back of the LCD before peeling off the bumpers, though, in case
      the screws are back there. On many players, the plastic bezel will come off the front,
      with the LCD anchored to the back, but some are the other way around. Often, the
      speakers are on the bezel, connected by wires, so remove it carefully to avoid tearing
      them. If the bezel won’t come off after you pull the screws, it either has internal snaps
      or is glued at the seam. Many of them are glued to prevent rattling from speaker
      vibrations. Use the snap-popping procedure described at the start of this chapter. If
      you find no snaps, try gently peeling up the bezel one edge at a time. The feel of
      separating glue will be unmistakable. Just remember not to let the bezel pop off hard
      or you’ll probably rip out those speaker wires. See Figure 9-4.


      MP3 Players
      These vary quite a bit, depending on who makes them. Some come apart easily, with
      accessible screws, while others are snapped together tightly and require a shim tool
      to separate. If you don’t see a way in, get on the Internet and look for disassembly
      instructions. You’ll find them, at least for the most popular players.
           Flash memory players are usually just one circuit board, with the display mounted
      to it. You’ll probably need to get to the troublesome headphone or power jacks, so you’ll




      Inverter




                   Figure 9-4     LCD with bezel off
                 Chapter 9      Entering Without Breaking: Getting Inside 181

have to remove the board unless all that’s required is a resoldering of the jacks’ contacts
and the board happens to be oriented such that you can do the job in situ. If the player
uses an internal battery, look for its connector and pull it before attempting repairs. As
with other products, having power applied when you’re working on the board can lead
to circuit damage.
     Hard drive players are a bit more complex, with a ribbon cable or two, and
possibly more than one board. In most cases, it’s best to remove the hard drive and
put it aside. These units are more likely to have separate display boards, too, so be
careful not to tear any ribbon cables when separating the case halves.


PDAs
These little handheld computers contain a remarkable number of parts, often on
more than one board. You’ll find a display, which also has the touch-sensitive digitizer,
a rechargeable lithium battery, a main circuit board crammed with chips, and a
switching-type power control and regulation system. Some PDAs have Wi-Fi and
cameras, too, making for even more cables and boards.
     The procedure for taking a PDA apart varies by maker, of course, but be prepared
for a challenge. The keyword here is “small,” so wear your magnifier. The multi-cup
layer approach will be useful, as will taking photos as you go. Always keep the screen
in mind, being careful not to scratch or press hard on it during your work. When
you lay the product on its face, check your bench first for screws or parts that could
damage the display.


Cell Phones
These are a lot like PDAs: lots of stuff in a very small space, with ribbons joining tiny
keyboards and displays. Plus, there’s an antenna on some that must be unscrewed and
removed before the case can be separated. Be sure to take off the battery and pull the
SIM (subscriber identity module) card as you begin. Be aware that the speaker magnet
can attract tiny screws.


Camcorders
Camcorders that record to memory cards are like digital cameras, so see that section
(next) for advice on taking them apart. Tape-based camcorders, analog or digital, are
a whole ’nother story. These can rival laptop computers for complexity. In some
ways they’re worse because they are so oddly shaped that boards and mechanisms
are crammed into nooks and crannies, making them hard to extricate. See Figure 9-5.
Plus, the mechanism is delicate and easily damaged during disassembly. Drag out the
cups and the digital camera.
    If you only need to clean the heads, open the cassette door and then remove its
two screws at the top. They may be under rubber covers. Take off the door and you
should be all set.
182   How to Diagnose and Fix Everything Electronic




                            Capstan motor     Head drum              Loading motor


                 Figure 9-5     Mechanism side of a camcorder


          For more extensive repairs, the machine will have to come apart. The typical
      camcorder body is in two pieces. Before you try to separate them, it’s best to remove the
      cassette door, as it often prevents getting that side of the body off. If the machine works
      enough to get the door open, pop it open, take out its screws and remove the door. If the
      door is stuck closed, remove the screws anyway and see if you can slide off the door.
      Most likely, it won’t budge, but you might be able to remove it once the case is loose.
          Look for arrows on the case indicating which screws need to come out. You may
      find screws all over the case, and most of them will have to go. On some cameras,
      various covers on the front and top have to come off because there are screws under
      them securing the shell to the chassis. Gently pull the two halves of the case apart,
      being careful not to get your fingers inside, where they could damage the mechanism.
      You’ll see ribbon cables all over the place, and you’ll have to disconnect a few once
      the machine is apart.


      Digital Cameras
      These are some of the hardest items to service. Most of today’s cameras are very slim
      and small, and the works are crammed in there tightly. Plus, cameras have lots of
      buttons, and some have sliding switches with plastic parts that fall off into oblivion as
      the case comes apart.
                Chapter 9      Entering Without Breaking: Getting Inside 183




          Figure 9-6     Inside a digital camera




     The case halves on many digital cameras are three-dimensional puzzles. To get
them apart, you may have to bend them around the edges slightly. On some, there’s a
plastic side piece surrounding the two halves, with tabs from each half fitting into it.
     Generally, the back comes off, with all of the circuitry and the LCD remaining on
the front. Be careful not to press on the LCD once the protective plastic window lifts
off with the back of the case. See Figure 9-6.
     As mentioned awhile back, digital cameras store the energy for the flash tube
in a large electrolytic capacitor. That baby can hold its charge of several hundred
volts or more for weeks after its last use. Usually, the cap is stuffed under the main
circuit board, next to the optical assembly. See Figure 9-7. Its leads, however, may
join the board just about anywhere. As you get the case apart, keep in mind that the
connection to the flash cap could be right under your fingers. I’ve gotten zapped
by digital cameras more often than by anything else. In addition to the danger to
you, discharging the cap through your finger or a tool can leak high voltage into the
camera’s sensitive circuits, causing instant, silent damage.


Laptop Computers
Laptops are among the most complex consumer devices and the toughest to take apart.
Talk about layers! You’ll want to use the cups and camera for these. Before you begin,
do an Internet search for disassembly instructions. Laptops are pretty trouble-prone,
184   How to Diagnose and Fix Everything Electronic




                       Figure 9-7     The evil flash capacitor: note the
                       voltage rating!


      and sites abound with help. Very often, the disassembly sequence must be followed
      exactly or the machine can be damaged. Plus, where you begin depends on what area
      you need to reach. Changing the hard drive might require a different procedure and
      degree of disassembly than would resoldering the power jack or swapping out the
      backlight inverter.
          Take off the battery before doing anything else. To remove the keyboard, look for
      snaps at the top. If you don’t see any, check for screws on the bottom of the machine.
      They’ll nearly always be placed such that they screw into the back of the keyboard
      near its top. I’ve seen a few near the middle, but none at the bottom, which usually
      has slots fitting into grooves on the top half of the case. Once the keyboard is loose,
      pull it up gently, keeping in mind that a ribbon cable connects it.
          If you’re trying to repair a backlight problem, check on the Internet to determine
      where the inverter is before taking anything apart. Sometimes it’s in the body of the
      machine, but it’s more likely to be in the LCD housing. If it is, you might not need to
      open the rest of the laptop at all.
          Most LCD housings are screwed together. Look for screws along the edges of the
      housing. If you don’t find any, check for cosmetic covers or bumpers on the front,
      near the bottom of the LCD. I’ve seen a few cases where screws were under the
      rubber bumpers at the upper corners, but not many. Often, those bumpers are pretty
      permanently attached and will tear if you try to pull them out. Then you find there’s
      nothing under them anyway. Before going that route, exhaust all other possibilities.
                Chapter 9      Entering Without Breaking: Getting Inside 185

Hidden snaps are common here too. Just avoid pressing on the screen; it’s easy to do
while pushing the seam along the edges, feeling for snaps.
    Probably the most common failure in laptops is a loose, intermittent power jack.
Repair is simple: just resolder the jack to the motherboard. Alas, getting to it isn’t
always so easy. For this one, you’ll need to take the case apart. Look for screws all
over the back, and keep track of their lengths when you take them out. Watch for
hidden snaps along the sides, and don’t bend the back too hard or you can break the
internal frame or the motherboard. Go easy.
    Some models mount the board to the internal frame, while others have it screwed
onto the back. Netbooks and lightweight notebooks don’t always have frames. Instead,
the major components are simply screwed to the back.
    If you’re lucky, the jack’s solder connections will be visible, and you can resolder
them without further disassembling the machine. If not, you may have to remove the
top half of the case. Be very careful of ribbon connectors going to the trackpad and
other items on the top half.
    I recommend not trying to take apart a valuable laptop if you’ve never done it
before, as the chance of wrecking it is substantial. Outdated machines are available for
very little, or even for free, from online resources and local computer recyclers. Get
one and practice on it.
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Chapter                    10
   What the Heck Is That?
   Recognizing Major Features

   W      hen you open a modern electronic product, the apparent complexity may seem
          overwhelming. Even pocket-sized gadgets can sport a surprising complement
   of goodies inside. MP3 players and GPS units, for instance, are miniature computers,
   with RAM, ROM and a microprocessor. Some even have hard drives.
        Older products, including those with double-sided circuit boards, usually had
   components only on one side. Not anymore! Thanks to the complexity and size of
   today’s gadgetry, parts are mounted on both sides. Components that stick up, like
   transformers and can-style electrolytic capacitors, are often relegated to one side so
   the board can fit flush against the case. Everything else is fair game. Transistors, chips,
   resistors and small inductors and capacitors may be anywhere. As you wend your way
   through a circuit’s path, you can expect to flip the board over numerous times.
        To find your way around in a box crammed full of parts, wires and boards, you
   need to become acquainted with what the major sections of the product look like,
   how components tend to be laid out, and how to follow connections from recognizable
   features back to those less obvious.
        Though the features vary a great deal depending on the product’s function, pretty
   much every device has a power supply section, an input section (or several), some
   kind of signal processing and one or more output sections. Let’s look at some common
   circuit sections and how to locate them.



Power to the Circuit: Power Supplies
   Everything has a power supply of some kind. It could be a pair of AA cells, a simple
   linear supply or a complex switching supply with multiple voltage outputs. Power supply
   problems account for many repairs, so recognizing the power supply section is vital.
       Batteries, obviously, are hard to miss. Battery-powered devices may have other
   power supply components as well, though, such as a switching converter to step up


                                                                                        187
188   How to Diagnose and Fix Everything Electronic

      a single AA cell’s 1.5 volts to a level high enough to run the product. Even when the
      battery voltage would be adequate on its own, a device may include conversion and
      regulation to ensure that a weak battery doesn’t affect the reliability of writing to
      memory cards or hard drives, both of which can be seriously scrambled by insufficient
      voltage during write operations. Digital cameras and laptops have such systems so
      they can operate properly until the battery is nearly dead and then shut down the
      device gracefully.
           To find voltage converters and regulation systems in battery-powered gear, look
      for small inductors and transformers. There are many varieties of them, but the
      telltale sign is metal. These things don’t look anything like transistors and chips. They
      may be round or square, but most are made from ferrite material, which is a darkish
      metal with a matte surface. Toroid (doughnut-shaped) cores are common. You may be
      able to see the wire wrapped around the core, but don’t count on it. See Figure 10-1.
           Along with these, keep an eye out for electrolytic capacitors. Although ’lytics can
      be sprinkled throughout any circuit, the larger ones tend to be congregated in or near
      power supply sections. Diodes and voltage regulators will be found there too. Most
      very small products don’t use enough current to require heatsinks on regulators and
      switching transistors, but larger devices usually do.
           It might seem natural that you could follow the wires from the battery
      compartment straight to the supply section, but it doesn’t always work out that way.
      Much of today’s gear uses transistor switching driven by a microprocessor to turn
      itself on and off, rather than a real power switch actually interrupting the current
      between the batteries and the circuit. There may be some distance from where
      the batteries connect to the board and the location of the voltage conversion and
      regulation circuitry. Those little transformers are your best landmarks.




                       Figure 10-1     Miniature inductors and
                       transformers
    Chapter 10       What the Heck Is That? Recognizing Major Features 189

     In AC-powered products, finding the power supply is a lot easier. Follow the
AC cord and it’ll get you there! Hard switches are still used in some AC devices, but
those with remote controls, like VCRs, DVRs, TVs and some projectors, use the same
microprocessor-controlled soft switching found in battery-op gadgets, so you can’t
count on the on/off switch’s being in line with the AC cord. Look for a transformer.
Switching supplies are pretty much standard now, but some products, especially high-
end audio amps and receivers, still use linear supplies for their essentially noiseless
operation. The transformer in a linear supply is a lot larger than the one in a switcher.
See Figure 10-2. In a switcher, look for a transformer like that shown in Figure 10-3.
     The power supply sections of pocket-sized devices are likely to be on the main circuit
board, while those in AC-powered devices are almost always located on a separate
board, with a cable feeding the output to the rest of the circuitry. See Figure 10-4.
     Once you’ve located the power supply, its major features are easy to spot. In a
switcher, the AC line will go through a line filter that looks like a small transformer,
then through a fuse, and on to the rectifiers, which may be separate diodes, a bridge
rectifier or a double diode, depending on the design. After that comes the chopper
transistor, which is probably heatsinked. Near it will be a large electrolytic capacitor
with a voltage rating in the hundreds. Then comes the transformer, followed by the
low-voltage rectification and regulation circuitry. At the very end, right by the output
wires, you’ll see the large electrolytic filter caps with significantly lower voltage
ratings than on the one next to the chopper.




                  Figure 10-2      Linear power supply transformer
190   How to Diagnose and Fix Everything Electronic




                    Figure 10-3    Switching power supply transformer




                     Figure 10-4    Power supply board. The chopper is
                     hidden by the transformer.
          Chapter 10      What the Heck Is That? Recognizing Major Features 191




Transformer                                                                         Transformer




              Figure 10-5     LCD backlight inverter. Note the plastic
              insulation over the transformers.


          In a linear supply, which has much less complexity, the AC line will go through
      the fuse and to the transformer. You won’t see a heatsinked transistor or a big
      electrolytic cap on the AC line side of the transformer. On the low-voltage side, you’ll
      find the rectifiers, regulators and filters.
          A special type of power supply is the voltage inverter, a step-up supply driven
      from the main power supply’s DC output. The inverter takes that low voltage and
      produces the high voltages required by LCD backlights. Inverters look like miniature
      switching supplies, which is basically what they are. The parts are much smaller,
      though, and you’ll see two transformers and two output cables in bigger displays
      with two fluorescent lamps behind the screen. Most designs put the transformers at
      opposite ends of the board. See Figure 10-5.



Follow the Copper-Lined Road: Input
      The input sections collect signals and feed them to the signal processing areas. The
      type of input circuitry present depends on the nature of the incoming signals. In radio
      and TV gear, input comes from an antenna or cable in the form of radio-frequency
      (RF) signals with strengths ranging from millionths to thousandths of a volt. The
      function of the input section is to amplify those very weak signals so further stages
      can separate out the desired one and demodulate it. The most common approach
      involves inductors (coils) resonating at the desired frequency. Analog tuners use a
      mechanically variable capacitor in parallel with the coil to change the frequency.
      Digital setups also use coils, but the tuning is controlled by the digital circuitry and
      accomplished with a varactor, which is a voltage-variable capacitor.
          TVs may also accept baseband video, which is the analog video signal without an
      RF carrier. Various flavors of baseband video include composite, in which the entire
192   How to Diagnose and Fix Everything Electronic

      signal is carried on one wire; S-video, in which the chroma (color) information is
      carried on separate wires from the luminance (brightness) signal; and component,
      which provides separate connections for red, green and blue. The input circuitry for
      each style of video is somewhat different.
          The easiest way to find video input circuitry is to follow the lines coming from the
      input jacks. In sets with multiple input types, which is most of them these days, the
      lines will go to some sort of switching circuitry first so the set can choose the desired
      signal. After that, the signal will be sent to the appropriate stages for amplification and
      preparation for further processing.
          Input can also be from a transducer, such as a laser optical head, phono cartridge,
      phototransistor, tape head or microphone. The signals from transducers may be
      very weak, as with magnetic phono cartridges, video heads and hard drive heads, or
      somewhat stronger, as with laser optical heads and phototransistors. Most of the time,
      the input circuitry will involve low-level amplification to prepare the signal for the
      signal processing sections. Because of their sensitivity to weak signals, input stages for
      transducers are often hidden under metal shields. Sometimes RF input stages are also
      shielded.


Shake, Bake, Slice and Dice:
Signal Processing
      Most products process a signal of some sort, be it analog or digital. Some kind of
      information is taken in or retrieved and massaged into whatever it is you want to
      hear, see, record, play back, send or receive. Much of the circuitry in any device is
      dedicated to signal processing. This is the little stuff, with lots of resistors, capacitors,
      transistors and chips. Digital circuitry is mostly chips, with a few bypass capacitors,
      and other small components that set operating parameters. See Figure 10-6.




                        Figure 10-6      Digital signal processing section
    Chapter 10      What the Heck Is That? Recognizing Major Features 193




          Figure 10-7     Analog signal processing section




     Analog signal processing circuits also use chips, but those tend to be small-scale,
with around a dozen leads, not a hundred. Analog sections also use more transistors
than do digital ones, and you may see variable capacitors, potentiometers and adjustable
signal transformers, especially in radio and TV receivers. See Figure 10-7.
     While many products are primarily digital, plenty of them combine analog
and digital functions, with a digital control system operating the analog sections.
Frequency-synthesized radios use analog stages to pick up, amplify and detect radio
signals, but their tuning is entirely digitally derived. Disc and MP3 players process
everything in the digital domain and then convert the results to analog for output.
     Digital control sections are recognizable by the large microprocessor chip with
lots of leads. Look for a crystal or resonator very close to the micro. If the product has
a display, it’ll probably be near the heart of the control section, as will a keypad or a
series of control buttons. See Figure 10-8.
     Even in all-digital devices, signal processing, output and power supply sections
look quite different from the control circuits.
194   How to Diagnose and Fix Everything Electronic




                                       Master control chip




                                                                 Clock oscillator

                       Figure 10-8     Digital control section




Out You Go: Output Stages
      Output stages prepare the signal for display, a speaker, headphones, a transmitting
      antenna, a print head, a motor, and so on. In many cases, a large part of this preparation
      is current amplification, to give the signal the oomph to drive a speaker, move a
      motor or push an RF signal for miles. The primary identifying characteristic of
      output stages is that they are larger than most of what’s around them. In some cases,
      like headphone amplifiers and small speaker drivers used in pocket radios and cell
      phones, the current required is low enough that the output stage may be quite small.
      Usually, however, more current is needed, so the parts are larger and better equipped
      to dissipate the higher heat.
           The style of output stage depends on what is being driven. Audio amplifiers
      driving large speakers with many watts will have sizable heatsinks for the output
      transistors or modules. If modules are used, they will be much larger than power
      transistors, with more leads. If discrete transistors are employed, they’ll range in size
      from around a postage stamp to perhaps 1 1/2 inches long. See Figure 10-9.
           Output components used to drive motors may have heatsinks, but they might
      not if the motor is small and doesn’t carry much load. Some motor drivers are just
      transistors on the circuit board. They’re likely to be a little larger than the others,
      though.
       Chapter 10      What the Heck Is That? Recognizing Major Features 195




                     Figure 10-9     These output transistors in a stereo
                     receiver are larger than average, but they have only
                     three leads, and their 2SC and 2SA part numbers
                     prove they’re transistors, not modules.


       The output circuitry for dot-matrix LCDs is integrated into the display itself, along
   the edges. This stuff isn’t serviceable; if a row or column goes bad, the LCD must be
   replaced. Simple numeric LCDs don't carry their own driver circuits. Instead, they're
   driven directly by the micro or by an external driver chip.



A Moving Tale: Mechanisms
   We’re heading toward a time when electronic products no longer include mechanical
   elements. Tape recording is already a dead technology. Eventually, everything will
   record to and play from memory chips, and even hard drives and optical discs will
   fade into obscurity.
       That time has not come yet, and some of today’s products still have mechanisms.
   Those in hard drives are sealed to prevent even the tiniest dust mote from crashing
   the heads into the platter, but the mechanics in optical disc players are readily
   accessible, and they cause enough trouble that you should get familiar with them.
   Digital Light Processor (DLP) TVs and video projectors use color wheel assemblies
   rotating at high speed. And if you’re servicing older technology like VCRs, tape-based
   camcorders, audio tape recorders and turntables, you’re going to get well acquainted
   with mechanisms and their peculiarities.
       Figures 10-10 to 10-13 show some mechanical sections you’ll encounter. For more
   details on specific mechanisms, see Chapter 14.
196   How to Diagnose and Fix Everything Electronic




                                                                 Capstan




                                     Head tip    Pinch roller


          Figure 10-10   Video head drum in a MiniDV camcorder




                     Figure 10-11   Camcorder capstan motor
Chapter 10     What the Heck Is That? Recognizing Major Features 197

             Back of spindle motor                   Back of laser head




                                     Sled motor

    Figure 10-12      Laser optical head sled assembly




             Figure 10-13      Color wheel assembly in a DLP
             video projector
198   How to Diagnose and Fix Everything Electronic


Danger Points
      As we discussed in Chapter 3, there are dangerous spots in many electronic products.
      While it seems obvious that AC-powered circuitry would be the most hazardous,
      don’t discount battery-operated gadgets as being harmless. Some products step up
      the battery voltage to levels that can give you a nasty jolt. Digital cameras, especially,
      generate hundreds of volts for their flash tubes, and it gets stored in a big capacitor
      capable of biting you weeks after being charged.
          Watch out for any exposed points connected to the AC line. Also, heatsinks are
      usually grounded, especially in audio amplifier output stages, but those in switching
      power supplies may not be. The heatsink on a switcher’s chopper transistor can be at
      hundreds of volts. Never touch it if the AC cord is plugged in. Even when the supply
      is unplugged, the heatsink may have a full charge from the electrolytic capacitor.
      Unplug the product and measure from the heatsink to the negative side of the big
      capacitor to see if any voltage is present. Remember, the AC side of a switcher is not
      connected to circuit ground, so measuring from the heatsink to circuit ground will
      show 0 volts, regardless of what’s actually there!
          The cases of metal power transistors in output sections can carry significant
      voltage as well. Avoid touching those without first measuring from them to circuit
      ground. Even 50 volts can do you harm if it’s applied across your hands, especially if
      they are wet or sweaty. If you’ve ever touched your tongue to the terminals of a 9-volt
      battery, you know how little it takes.
          The output connections of backlight inverters can be at 1000 volts or more. Keep
      away from them when the device is powered on.
          VCRs, DVD players and some other products use small fluorescent displays.
      Lighting those up requires a few hundred volts, so beware of their connections.
Chapter                   11
  A-Hunting We Will Go:
  Signal Tracing and Diagnosis

  N    ow that you have the unit open and ready for diagnosis, it’s time to apply the
       ideas we’ve been examining and put that oscilloscope to good use. Locating the
  trouble is the heart of the matter and much of the battle. The general approach is
  to reduce your variables to eliminate as much circuitry as possible, concentrate on
  what seems a likely problem area, take some measurements, apply a little logic, and
  gradually narrow your focus until you reach the bad component.
       Where to begin? That depends on what symptoms are being displayed. In order
  from least functionality to most, here are some good ways to pick a starting point.



Dead
  As we discussed awhile back, dead means nothing at all happens when you try to turn
  the unit on. If that’s the case, head straight for the power supply. Check the fuse first.
  If it’s blown, assume something shorted and blew it. The short could be nearby, in the
  rectifiers, the chopper transistor or its support components, or it could be somewhere
  in the circuitry being powered by the supply, far from where you’re looking.
        If the supply feeds the circuitry through a cable, disconnect it. Replace the
  fuse and try applying power. Does the fuse blow again? If so, the problem is in the
  supply. If not, the fault could still be there, but more likely a short in the circuit being
  powered is drawing too much current and popping the fuse.
        Some power supplies have small, low-current sub-supplies for standby operation,
  so they can keep enough circuitry alive to respond to remote-control commands or soft
  switches. The main supply turns on only when commanded to do so by the product’s
  microprocessor. With the cable disconnected between the supply and the rest of the
  circuitry, the micro can’t command the supply’s main section to start up. If the short is
  in a part of the supply not running while in standby, the supply will appear to be okay
  and will not blow the fuse, confusing the matter of where the short lies.


                                                                                        199
200   How to Diagnose and Fix Everything Electronic

           If the fuse is not blown but the unit still does nothing, take a good look at the
      electrolytic capacitors in the supply and also in other areas of the unit. See any with
      even slight bulges on top? If so, forget about continuing your exploration until you’ve
      replaced them. By the time a cap bulges, it’s pretty far gone, with perhaps 10 percent
      of its original capacitance left. Its equivalent series resistance (ESR) will be way up as
      well. Very likely, replacement of the bulging parts will restore operation of the unit.
           If you don’t see bad caps, check the supply’s output voltages with your DMM.
      Find circuit ground on the output side (never on the AC input side of a switcher!)
      and hook the black lead to it. In a unipolar design, the negative output lead is almost
      always ground. If the supply is bipolar, there will be positive, negative and ground.
      Even if the supply provides several output voltages, one ground serves them all,
      although multiple wires may be connected to it.
           In a metal-encased product like a disc player or VCR, the metal chassis should
      suffice as long as you can find a spot that’s not painted over. In a pinch, you can
      usually use the outer rings of RCA jacks in audio/video gear. Choose an input jack,
      not an output jack, so you don’t risk shorting an output if your alligator clip makes
      contact with the jack’s inner conductor.
           Many supplies have markings for the voltages on the boards, right next to where the
      output cable plugs in. If so, see if the voltages are there and are fairly close to their rated
      values. Don’t worry if a line marked 5 volts reads 5.1. If it reads 4 or 6, then something’s
      out of whack. When the voltage is too high, the problem will be in the supply’s
      regulation. When it’s too low, it could still be a regulator issue, but a short elsewhere in
      the circuitry might be pulling it down. If the voltages are okay, the supply is probably
      fine. If they read zero, it might still be fine and just isn’t being turned on, as described,
      but it’s quite possible it isn’t working. If the supply is turned on and off by the unit’s
      microprocessor, there still has to be some voltage from the standby supply to run the
      micro or it couldn’t send a signal to start the main supply.
           A product running off an external AC adapter might not blow its fuse even when
      seriously shorted. Most modern AC adapters are switching supplies. A well-designed
      one will go into self-protect mode, sensing the excessive current draw and shutting
      down. Usually, it’ll restart every second or so, pumping some current into the device
      and then stopping again because the load is outside the normal range, never staying
      on long enough to melt the wire inside the fuse. Even the primary-side fuse in the
      adapter may survive, for the same reason. I once fixed a laptop computer’s AC adapter
      that had a shorted output cable but never blew its fuse. The adapter’s self-protect
      mode saved the fuse and the rest of the supply as well. The good fuse confused
      my diagnosis attempts until I considered the self-protect mode, checked the cable
      and found the short. After cutting off the bad section of cable and resoldering the
      remaining good length, I plugged the supply in and it worked fine.
           Internal power supplies in AC-operated devices may also survive shorts without
      blowing their fuses, but they usually aren’t as well-protected as external adapters, and
      the fuse blows.
           If you have a working supply but no operation, head for the product’s microprocessor
      and check for an oscillating clock crystal or resonator. If you find no voltage at all,
      there could be a little sub-regulator on the board to power the micro, and it might be bad.
    Chapter 11        A-Hunting We Will Go: Signal Tracing and Diagnosis 201

If you see voltage there (typically 5 volts, but possibly less and very occasionally more)
but no oscillation, the crystal may be dead. Without a clock to drive it, the micro will sit
there like a rock. If you do see oscillation, check that its peak-to-peak (p-p) value is fairly
close to the total power supply voltage running the micro. If it’s a 5-volt micro and the
oscillation is 1 volt p-p, the micro won’t get clocked. If you have power and a running
micro, you should see some life someplace.
     Lots of products include small backup batteries on their boards. See Figure 11-1.
These batteries keep the clock running and preserve user preferences. Loss of battery
power causes resetting of data to the default states but doesn’t prevent the product
from working. In some cases, though, a bad battery can indeed stop the unit from
turning on. I’ve seen laptop computers that wouldn’t start up unless the bad backup
battery was disconnected.
     The batteries may be primary (nonrechargeable) lithium coin cells or secondary
(rechargeable) types. Often they’re soldered to the board. Primary types can be
replaced with standard lithium coin cells of the same type number and a holder,
as long as the arrangement will fit. You can even use bigger or smaller cells, since
they’re all 3 volts anyway; smaller cells just won’t last as long. Secondary cells need
to be replaced with the same type as the original, and those are not easy to find.
Most likely, you’ll have to try to order one from the manufacturer. Don’t replace a
rechargeable cell with a primary type, because the applied charging voltage will cause
the nonrechargeable cell to burst.
     If you’re suspicious of a bad backup battery, measure its voltage. Should it be very
low, disconnect the battery and see if the product comes to life.




                  Figure 11-1      Soldered rechargeable backup
                  battery in a digital camera
202   How to Diagnose and Fix Everything Electronic


Comatose or Crazy
      This situation is trickier. When the unit turns on but is completely bonkers, with
      random segments on the LCD and improper or no response to control buttons, that
      usually indicates one of three things: power supply voltages are way off (probably too
      low), there’s lots of noise on the power supply lines or the digital control system is
      seriously whacked out. Check the supply voltages first. If they look close to what they
      should be, scope the noise. Using your scope’s AC coupling, look at the supply’s DC
      output lines. There shouldn’t be more than 100 mv (millivolts) or so of junk on them. If
      you see much more than that, you can expect to find bad electrolytic caps in the supply.
           The most likely culprits are the caps right on the output lines. A good electrolytic
      will smooth out that noise, so its presence tells you the cap is not doing its job. Even if
      the part doesn’t bulge or exhibit obvious leakage, try replacing it or temporarily putting
      another cap of the same value across it. Be sure to get the polarity right when you
      do this! And, of course, shut everything down first and verify that the existing cap is
      discharged. Don’t worry about the two caps adding up to more than the correct value;
      some extra capacitance on a power supply line will only lead to better filtering. If the
      noise drops dramatically, change the cap, regardless of whether proper operation was
      restored when you jumped it with the good one. It might still be bad, but there could
      be others that will have to be changed before the unit will work again. If the noise
      drops only a small amount, then the original cap is okay, and you’re just seeing the
      added capacitance smoothing things over a little bit. The trouble is elsewhere.
           When you’re sure the power supply is working properly, go to the microprocessor.
      Check it for clocking, just as with a dead unit. If the clock looks normal, it’s possible
      that the reset circuit, which applies a pulse to the micro’s reset pin when power is
      first applied, isn’t working, so the micro isn’t starting up from the beginning of its
      program. Many reset circuits are nothing more than an electrolytic cap between
      the positive rail and the reset pin, with a resistor going from there to ground. When
      the cap is discharged and the unit is turned on, the change in the cap’s charge state
      momentarily lets a spike through to the reset pin until the cap charges up and blocks
      the rail’s voltage. If that cap has dried out or leaked, the reset pin won’t get tripped,
      and the micro will start up at some random place in its firmware, resulting in digital
      insanity. If you can find the reset pin, disconnect power, scope the line going to the
      pin and then reconnect power. You should see a pulse. If you don’t, try turning on the
      unit. Still no pulse? The reset system isn’t working. Try replacing that capacitor. Or, if
      there’s a transistor, diode or other circuit generating the reset pulse, work backward,
      from its output to its input, scoping your way until you either find a pulse or locate its
      missing source.



Alive and Awake but Not Quite Kicking
      This is where your sleuthing skills really get a workout. The unit powers up and
      responds properly, but some function doesn’t work. Perhaps a backlight is out, or a
      disc player has trouble reading discs, or a VCR plays only in black and white. Maybe
       Chapter 11      A-Hunting We Will Go: Signal Tracing and Diagnosis 203

   a projector turns on but the lamp won’t strike. Figuring out these kinds of failures can
   take much more work than does troubleshooting the dead and semiconscious types.
        After checking for the usual power supply issues, take an especially careful look
   on the board for bulging or leaky electrolytics. Change any that don’t look normal.
   Sometimes ’lytics can be bad without showing physical signs. Scope them. As a rule,
   any electrolytic with one end tied to ground should have very little besides DC on the
   other end. Especially if you see high-frequency elements to the noise—it’s fast or has
   spiky edges—jump that cap with another one and look at the noise content again.
        If these simple checks don’t turn up anything, it’s time to go snooping. Is the
   problem at the input side, the signal processing midsection or the output stages?
   With audio gear, listen for a slight hiss from the speaker. If it’s absent, the problem
   is likely to be in the output stages, because those will produce a little noise even if
   what’s feeding them doesn’t work. The speaker itself could be bad too; check with
   headphones or scope the output lines going to the speaker. With video equipment,
   there might be some noise on the screen, indicating that the stages driving it are
   working. With other kinds of devices, it could be harder to tell.
        Items that move, like laser heads, print heads and swing-out LCD viewfinders on
   camcorders, have plenty of problems with broken conductors in their ribbon cables.
   If a moving part misbehaves, look at its ribbon with a magnifier. Even if you can’t
   see a break, check the integrity of each line on the ribbon using your DMM. The
   very thin, flexible ribbons with black printed conductors rarely break, but the slightly
   thicker green ones with copper conductors are quite prone to fractured lines. Even
   if the conductor side looks black, check the fingers on the ends, where they make
   contact with the connector’s pins. If they’re copper-colored or look like they’re coated
   with solder, check the ribbon carefully.



Sometimes Yes, Sometimes No
   Want to give a tech nightmares? Sneak up behind the poor sap and whisper the word
   “intermittent.” Watch for neck shivers and twitching muscles. Nothing is more difficult
   to find than a problem that comes and goes. Naturally, it goes when you look for it
   and comes back after you’re done.
        Thermal intermittents represent the easier-to-cure members of the genre. If a
   product works when cold but quits after warm-up, or the other way around, at least
   you can control those conditions while you hunt the trouble. Your most powerful
   weapon is a can of component cooler spray. After the operating status changes state,
   grab that can and get ready to spray. You don’t want to spray the entire unit part
   by part, so concentrate on the kinds of components most likely to cause thermal
   problems: those that get warm. Voltage regulators, power transistors, graphics chips
   and CPUs all generate lots of heat and should show sudden change when you blast
   them with the spray, if they’re the troublemakers. Spray small parts for about a
   second. Big ones may require as long as 5 seconds. Avoid hitting your skin, as the
   spray can cause frostbite. And, of course, keep your eyes away! I keep my face at least
   12 inches from any part I’m spraying.
204   How to Diagnose and Fix Everything Electronic

            Now and then, thermal intermittents occur in small-signal parts too. If a transistor
      is leaky, it may get warmer than would a properly functioning part, driving itself
      into thermal weirdness. I remember one radio transceiver (transmitter/receiver) that
      would peg its signal strength meter to the far right when turned on, and no signals
      could be heard. As it warmed up, the meter would slowly drop to normal, and signals
      would gradually rise until they came in loud and clear. Knowing that the meters in
      radios are driven by the automatic gain control (AGC) circuit, I went hunting in that
      area. Spray can in hand, I finally found it: a small, garden-variety NPN transistor,
      leaky as could be. Oddly, it was shorting when cold and started working properly as
      the current through the short warmed it. Twenty-five cents and three solder joints
      later, the receiver was back to normal.
            Electrolytics can be thermal as well. Of course, a cap shouldn’t get hot in the first
      place. Some get a little warm, especially in switching supplies, but a really hot cap
      means there’s a current path through it, so it’s acting like a resistor. In other words,
      it’s leaking.
            If you blast any part and the unit starts or stops working, that’s a pretty good
      sign you’ve found the bad component. When parts are crammed together, you may
      think you’re hitting the right one, but a little bit of the spray is splattering on another
      component that’s actually the culprit. Sometimes you have to spray a few times from
      different angles to be sure which part you’re really affecting, letting the suspect and
      the components around it warm up again between sprays.
            Solder joints can be thermal too. Sometimes when you spray a component and
      it starts (or stops) working, the real problem is at its joints. Before changing the
      part, always check the soldering and touch it up if you’re not sure. Test again before
      replacing the component.
            Mechanical intermittents are the hardest problems of all to find. When a machine
      exhibits symptoms by being tapped on, turned or tilted, there goes your night. And
      the next, and the next, probably.
            Vibration or position-sensitive intermittents are caused by bad connections.
      They could be cold solder joints, circuit board cracks, dirty connectors, bad layer
      interconnects or, rarely, fractures inside components. Tap around, see what trips the
      symptoms, fix it, done. Seems simple enough, right? How hard could this be? Plenty.
      These kinds of intermittents tend to be very sensitive, causing malfunction no matter
      where you tap or flex. The basic search technique is to press and tap ever more gently
      as you home in on the problem area, hoping to localize the effect until you get down
      to one spot. Alas, even when you barely touch the board, the part flexing or vibrating
      may be far from your point of contact.
            Circuit board cracks are rare these days, except when a product has been dropped.
      Most cracked boards stop working completely, but now and then a cracked trace will
      have its edges touching just enough to cause a vibration-sensitive intermittent. Far
      more common are bad layer interconnects, especially the conductive glue variety.
      Even plated holes can cause intermittents, but not very often. Conductive glue may
      look fine but not be making a solid connection with the upper or lower foil traces.
            If you suspect a bad interconnect or a cracked trace, jumping with wire, even
      temporarily, will settle the question. If an interconnect isn’t solid with an inner layer
       Chapter 11      A-Hunting We Will Go: Signal Tracing and Diagnosis 205

   of the board, it can be tough to figure out where the jumper should go unless you
   have a schematic. Because the connection isn’t totally lost, though, you can use your
   DMM to trace to other components. Keep an eye on the actual resistance to avoid
   reading through other parts and thinking they’re connected when they’re not. Expect
   to see some resistance. After all, that’s the problem, right? If you see what looks like a
   connection, tap on the board and see if the reading changes. Remember that a DMM
   doesn’t respond very fast. An analog VOM’s needle will bounce, which is more useful
   in this case.
        Many products use the chassis or case as circuit ground, with grounding pads on
   the board making contact when it’s screwed down. As the device ages, loosened screws
   and oxidation degrade those critical connections, leading to intermittent behavior.
   In a unit more than 5 years old, check those pads even if the screws are tight. If the
   pads are dirty or oxidized, clean them up and see if that cures the symptoms. In a
   newer item, all should be well unless the screws are loose or the unit has lived in an
   especially corrosive environment like a boat.
        Probably the most frustrating intermittent of them all is when the unit works just
   fine until you close up the case. Then it either won’t work at all or it becomes motion-
   sensitive. You open the case back up again and the little monster works perfectly.
   Arggh!
        To get to the root of one of these seemingly intractable dilemmas, consider what’s
   happening when the case is closed. Look at the inside of the case and visualize where
   it will press on the board, on wiring and on ribbon cables. Some cases hold down the
   corners of the circuit board, flexing it when the screws are tightened. Experiment
   while the case is open, trying to re-create the problematic conditions. Most of these
   can be solved, but I’ve run into a couple I couldn’t straighten out.
        If the board isn’t too sensitive to probe without altering the symptoms, use normal
   signal tracing techniques to locate the intermittent. If everything you touch disturbs
   the intermittent, it’s very difficult to make sense of what you see on the scope.



To and Fro
   Some techs like to work backward most of the time, starting at the output stages and
   hunting back toward the input area, looking for where the signals stop. Others prefer
   to start at the input and see where things get lost. What’s the best method?
        Either way may be appropriate. The output-to-input approach is especially useful
   when there is an output signal but it’s not normal. Very often, such problems arise
   in the output stages and their drivers, so why start way back at the input and scope
   through stage after stage to get there? If you see a normal signal feeding the output
   stage, you’ve pretty much nailed it without a lot of hunting.
        Digital devices offer a powerful clue to help you decide your direction of attack: is
   the time counter moving? If so, the device is receiving data, be it off a disc, a memory
   card or internal memory, and at least the heart of the digital section is working. So,
   start at the output and work your way back. If the counter is not progressing, head for
   the input area and find out why not.
206   How to Diagnose and Fix Everything Electronic

           With items like RF receivers, you may have normal audio hiss or video snow but
      no reception. Since the path through a receiver is fairly complex, with oscillators,
      tuned IF amplifiers and demodulation stages, it makes more sense to start at the input
      and work forward. At some point, you’ll discover a missing oscillator or a dead IF or
      demodulator stage, with corresponding loss of signal.
           If you are going to work backward, be certain you have a valid input before you
      start looking for it way down the line! Just because you plug an audio source into a
      stereo receiver, for example, doesn’t mean it’s getting to the amplifier board. There
      could be an issue with the input switching, or your connecting cable might be bad.
      Check the input signal at the board to be sure. Camcorder won’t play in color? Are
      you sure the tape you’re trying to play has color? If recorded on the same machine,
      it could be that the fault is in record, not play. Use a known good tape, or verify the
      existing one by playing it on another machine.
           In many cases, a hybrid approach is the most effective. Start at the output and
      work back a few stages. If you can’t find the signal, go to the input and work forward.
      In complex systems with multiple inputs, such as servos, check the inputs to be sure
      they’re all there, since one missing signal will turn the whole thing into a mess.



All the World’s a Stage
      Always remember the all-important organizational concept of the circuit stage. You’re
      not going to scope every darned component in the device. Instead, you’ll focus on a
      particular area in the unit and look at it stage by stage.
           Test points are very handy. With a schematic, you can look up TP204 and find out
      what it’s supposed to show. Even without a diagram, you can often guess the signal
      being tapped from the waveform when you scope it. Sometimes you really get lucky,
      and the test points are labeled for function, in addition to their call numbers. You
      might see “reset” or “trk gain” (tracking gain). Checking those points and interpreting
      their signals can save a heck of a lot of work. If a test point at the end of a chain of
      stages shows the expected behavior, there’s no need to scope each stage; they all have
      to be working.
           Test points for digital signals like “reset” may show a line above the word. That
      means “not reset,” which is tech-ese for “the signal goes low to initiate the reset,
      not high.” When there’s no line, the signal should go high, but don’t count on that.
      Some manufacturers don’t bother adding the line. If you see one, though, the signal
      definitely goes low.
           Only when you find a nonfunctional stage is it worth trying to discern what part
      in the stage is preventing it from working. In the vast majority of cases, that part
      will be either an electrolytic capacitor or an active element: a transistor or a chip.
      With a few exceptions, like crystals and high-voltage transformers, other components
      that may have gone bad are probably victims of having had too much current pulled
      through them, and are not the perps themselves.
           Diodes, rectifiers and zeners represent a special case. Though they’re not active
      in the sense of having gain, they are semiconductors susceptible to the same kinds of
       Chapter 11       A-Hunting We Will Go: Signal Tracing and Diagnosis 207

   failures found in transistors. Most techs think of them as active elements and check
   them before looking at more reliable components like resistors, coils and ceramic
   capacitors.
        Zeners, which dissipate excess power as heat, are particularly prone to being
   open. Replacing a marked zener is no big deal because you can look the value up by
   its part number. Unmarked zeners present a much bigger problem if you don’t have
   the schematic. What was the zener voltage supposed to be? You’ll never know for sure,
   but you can make an educated guess.
        First, the zener voltage will be less than what you’re measuring at the blown zener,
   since the whole point of a zener is to reduce the voltage to the diode’s breakdown
   rating; the part does nothing when the voltage is below that value. Theoretically, the
   zener voltage could be as little as a volt less than the applied voltage, but expect it to
   be at least a few volts less. Look for electrolytic caps in whatever circuit the zener
   regulates. The zener voltage will be less than the caps’ voltage ratings. Again, it should
   be at least a couple of volts less, since few designers are foolish enough to run ’lytics all
   the way up at their ratings.
        Though there’s a wide range of zener values, many circuits operate on 5, 6, 9 or
   12 volts, and it’s reasonable to expect most of the zeners you find to be one of those
   values. Microprocessor circuits commonly use 5 volts. In audio power amplifiers,
   zeners are used to establish bias, and calculating the correct value isn’t simple. Luckily,
   you should have another channel in which to measure the voltage across its good zener.



Check, Please
   When you find a stage that isn’t functioning, don’t be too quick to indict it. First, be
   sure it’s receiving the power and signals it needs to do its job. You really can’t blame
   the poor transistor if it’s not getting voltage, if its bias is way off, or some other stage
   isn’t turning it on or providing proper input.
        Unfortunately, cause and effect aren’t always so clear. When a signal or a voltage
   appears to be missing, it could be that the stage is receiving it but a bad part is
   shorting it to ground. Or, a coupling component could be open, preventing the signal
   from getting to the active element. How to tell?
        If there’s a resistor between the source of the signal and the stage you’re examining,
   check on the other side of it. The current limiting of the resistor isolates the far side
   from anything happening at the suspicious stage. The bigger the resistance value, the
   more isolation you can expect. A few ohms won’t give you much isolation—signals will
   be about the same on both sides—but a kilohm or more sure will. You should be able to
   see something of the original signal on the other side, even if it’s reduced in amplitude.
   If not, then the stage on that side isn’t sending it, and you need to move your hunt to
   that part of the circuit.
        A capacitor can provide isolation for AC signals, but how much depends not only on
   the size of the capacitor but also on the frequencies involved. The higher the frequency,
   the smaller a capacitor it takes to pass it, so the less isolation you get for a given
   capacitance value.
208   How to Diagnose and Fix Everything Electronic

           Once you’re certain the correct conditions have been met, it is reasonable to
      conclude that you have a bad component, and it’s time to start checking them. Unless
      you see a leaking cap, head for the active element first. Even if you find a burned
      resistor, you can bet the active element pulled too much current through it.
           Sometimes parts can be tested while they’re in the circuit, but usually the effects
      of other components will confuse the measurement, and you’ll need to pull the
      suspect part before you check it. Two-legged parts need only one lead disconnected.
      If one lead goes to ground, remove the other one and leave the ground side connected.
      It’s easier, since ground lands are typically the biggest and hardest to desolder. Also,
      you can leave one test lead connected to circuit ground and will have to connect only
      one lead to the component.
           Wick the solder out of the hole and bend the part up on its other lead. For a
      three-leaded component like a transistor, pull two leads, and be sure one of them is
      the base or gate lead, so other parts can’t influence the sensitive terminal with added
      capacitance or noise pickup via the rest of the circuitry.
           With some parts, especially electrolytics, it might not be possible to bend the
      component on one lead if the leads are too short. If you can wick the hole thoroughly
      enough that the stub of the lead moves freely within it, testing is possible without
      pulling the component. With your test probe, push the stub away from the walls of
      the hole, watching the test results as you do so. The readings should make it apparent
      when contact with the rest of the circuit has been broken.
           You’ll be amazed at how many times you’re absolutely sure you’ve found the
      problem, you pull the part, and it tests out fine. It can be frustrating, but that’s just the
      nature of the repair experience. Eventually you’ll nail it, and it feels really great when
      you pull the fifth part you were certain had to be the culprit, and it actually is!



When All Else Fails: Desperate Measures
      No matter how good a sleuth you are, sometimes nothing works. You pull part after
      suspicious part and they’re all good. You’ve been at it for hours, you’re out of ideas
      and desperation sets in. Welcome to the technician’s club! It happens to all of us once
      in awhile. Here are some desperation techniques to try. They may seem crazy, but
      they’re better than giving up and tossing the unit on the junque pile. Now and then,
      they actually save the day.


      Shotgunning
      This is as old as electronics itself. When you have an intermittent connection you
      just can’t find, solder them all! Back in the days when circuits had a few dozen parts,
      shotgunning was easy and quick. Today, with hundreds of joints on every board,
      shotgunning can take quite awhile, and it’s not feasible with laptops and other extremely
      dense, complex products.
           Start with an area you think is causing the trouble and hit every joint in it. If it
      doesn’t work, keep on going. Don’t be surprised if you wind up redoing every joint
    Chapter 11      A-Hunting We Will Go: Signal Tracing and Diagnosis 209

in the entire product, and it still won’t work. Frankly, shotgunning is rarely successful;
the real problem always seems to be something obscure that gets missed. Now and
then, though, luck prevails and the symptoms disappear. Don’t get too excited—you
might have only wiggled the actual bad connection, and the problem will return…
typically right after you tighten the final case screw. Once in a great while, I’ve seen
shotgunning result in a real repair.


Current Blasting
This one has a little more basis in sanity. It’s useful only when you have a dead
short across the power supply rails somewhere on the board, but you can’t find it.
Especially on today’s digital boards, there are lots of little bypass capacitors from Vcc
(the positive supply rail) to ground. Now and then one of them shorts. You see the
short no matter where on the rail you probe with your ohmmeter, so it’s impossible
to deduce which of the 50 little caps might have become a zero-ohm nightmare, and
pulling them all to test them presents too much risk to the board. Plus, it’d take hours,
and you can’t be sure the short isn’t in some other component anyway.
     There exist exotic ohmmeters that read ultra-low resistance values down in the
milliohms (thousandths of an ohm), allowing you to follow the traces and see when
you’re approaching the short. You don’t have one of these babies, though, and neither
do I. Even if you did, it would be hard to check the whole board and make sense of
what you see.
     There’s a faster, easier way. You probably have a high-current power supply,
either on your bench or perhaps in a discarded desktop computer. To perform current
blasting, you need a supply of the same voltage as the product’s supply. Many of these
direct-short situations involve 5-volt digital boards, so a computer supply is a good
choice. You need a lot of current—perhaps 20 amps or so. Your little 2-amp variable
bench supply won’t do it, but a PC supply has the required oomph.
     If at all possible, disconnect the product’s own power supply so you won’t be
feeding voltage into its output. That’s usually okay, but some voltage regulators can be
damaged when their output voltage exceeds their input voltage (which will be zero in
this case), so it’s best to avoid having them connected during this maneuver. If there’s
a removable fuse between the supply’s output and the rest of the board, pulling that
should do it. Otherwise, yank the connector or unsolder the positive wire.
     With the hefty supply turned off, connect its +5V and ground wires to the
board’s supply rail and ground traces. Naturally, + goes to +. Make sure you’re
past any fuses, because this procedure will blow them. This is one time you don’t
want protection. Turn on the supply and wait. After perhaps 30 seconds, the shorted
part will start smoking and burning, because pretty much all the supply current is
going through it. As soon as you see the smoke, kill the supply; you’ve found the
bad component. If the part is something nonessential, like a bypass cap, the product
may start working as the current cooks the cap. I’ve seen the voltage rise high
enough to start up a device even before the short clears. It’s amazing what enough
current will do.
210   How to Diagnose and Fix Everything Electronic

           This procedure will work with other voltages too, of course, as long as you have a
      supply that can source a lot of current.
           Some caveats: it’s possible the board’s traces could melt before the shorted part
      gets hot enough to smoke. I haven’t seen that happen, but it could. On a dense or
      multilayer board, a melted trace could prove disastrous. Also, the big supply must not
      have self-protection or it’ll refuse to dump lots of current into a short. I’ve had good
      luck with desktop PC supplies; they are very sturdy and don’t mind the overcurrent,
      at least not for the period of time required. They also don’t seem to have self-
      protection circuitry.
           If nothing gets damaged, current blasting pretty much always works when the
      shorted part is a capacitor. Sometimes the short is inside a chip, and the high current
      instantly blows it open. Nothing smokes, you don’t know where the short was, the
      device still doesn’t work, and you’re no better off than when you started. Still, it’s a
      useful technique and it beats just giving up.
           I once fixed a really nice little hard-drive MP3 player that way. It had a dead short
      across the power supply input jack, and the board was too small and dense for me
      to consider trying to pull parts and test them. It was a 5-volt unit, so I hooked up a
      computer supply and hit the switch. In 10 seconds a surface-mount electrolytic cap
      right next to the power jack lit up like a tiny light bulb. I changed it and the unit came
      back to life. I saved a rather expensive digital piano with current blasting too. The
      shorted component was a tiny bypass cap near the microprocessor. I’d never have
      found it any other way.


      LAP Method
      This is the craziest last-ditch method of them all, but it has worked for me on rare
      occasion. LAP stands for “least accessible place.” Where’s the hardest place to reach in
      the entire product? If every other option has been exhausted, head there and suffer
      through whatever it takes to examine that difficult area. After a few LAP successes, I
      used to wonder how this could possibly be real. Was some cosmic force hiding things
      from me? Was there a ghost in the machine with a bad sense of humor?
           The more reasonable explanation was that it was the one place I hadn’t been yet!
      It seems like no matter how hard we try to check everything, there’s always some
      forgotten nook so inaccessible that we don’t even notice it, or we subconsciously avoid
      it. And, if it’s the last possible place, the trouble just might be there.
Chapter                    12
   Presto Change-O: Circuit Boards
   and Replacing Components

   O      nce you’ve found a component you want to test, or one that’s obviously blown,
          you need to remove it from the board. Back when all components were mounted
   on leads pushed through holes in single- or double-sided circuit boards, removal was
   easy. A little solder wick or a pump of the solder sucker, and the holes would clear.
   After that, all you had to do was pull.
         Sometimes the process is still like that, but now there’s much more variety of
   component styles requiring different removal techniques, and multilayer boards have
   complicated the situation. Component removal ranges from trivial to maddening, and
   it’s easy to destroy the circuit board when a recalcitrant part simply refuses to budge.
         Unless both sides of the board are accessible, you’ll have to remove it from the
   unit before you can desolder anything with leads poking through the board. Either
   way, first make sure power is disconnected. I always look at the AC plug before
   beginning to unscrew a board or desolder components, just so I know the plug is
   definitely lying loose. Even if I remember having pulled it, I take another look.



Through-Hole Parts
   Many larger components still use the old wire-through-the-hole mounting technique.
   To remove power transistors and other through-hole parts, the solder must be sucked
   out of the hole, or the lead has to be pulled out while the solder is molten. Clearing
   the hole is preferable. For small joints, use solder wick, as described in Chapter 6.
   Place the end of the wick on the joint you want to desolder, and then press the iron’s
   tip on the other side. Hold it there for about 20 seconds, and the solder should flow up
   into the wick. See Figure 12-1.
        This doesn’t always work, though. Sometimes the solder won’t flow well enough
   to clear out the hole. The usual reason is insufficient heat, but transferring the heat to
   the joint is an issue too, as is thermal absorption by large copper lands. If you can’t get


                                                                                        211
212   How to Diagnose and Fix Everything Electronic




                        Figure 12-1      Using solder wick



      a small land’s hole to clear, try adding some fresh solder, and then wick it out again.
      Boards manufactured with lead-free solder don’t desolder well. Adding leaded solder
      to a lead-free joint lowers the existing solder’s melting temperature, making removal
      easier.
           The wick absorbs some heat too, so it takes a hotter iron to desolder a lead than
      it does to solder it. Plus, to remove a lead requires wicking out all of the solder in the
      hole. With thick or multilayer boards, some of it may be a millimeter or more away
      from the heat source, making the solder hard to melt.
           Desoldering is complicated by the increased thickness of multilayer boards
      and their extra heatsinking effect from internal foils contacting the copper coating
      inside the holes. Applying enough heat to wick the solder out can destroy the board.
      To remove a stubborn lead from a multilayer board, it’s best to heat one side while
      pulling the lead out on the other, and then clear the hole after the lead is gone. Even
      then, you may struggle with it and be tempted to reach for the big soldering gun.
      That’s too much heat for small boards, and it can deform them and break internal
      connections in multilayer boards, wrecking the device. See Figure 12-2.
           Large lands used for power supply and ground buses create a heck of a heatsinking
      effect. It can be quite frustrating trying to get them hot enough to melt and clear the
      solder. The big gun might be called for here, but it’s still possible to trash the board
      because there may be other lines running over the big land inside the layers. Heating
      up the big land can break them or short them to their adjacent layers.
           If a part won’t come out no matter how hard you try, it’s a lot safer to clip the
      leads and solder in the new part without clearing the holes. Clip the new part’s leads
      close and solder them to the residual solder in the holes. You should be able to heat a
      hole enough to make a good joint, even if the solder at the far end of the hole never
      melts. If there’s room, you can leave a little of the old part’s leads and solder to those.
 Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 213




              Figure 12-2    How not to do it! Excessive heat destroyed this
              multilayer board.


         Sometimes you can’t get to the leads to clip them. On most electrolytic caps, the
    leads are under the parts, unreachable with any tool. The easiest way out is just to
    chop off the component near its base with a pair of wire cutters. Then you can clip or
    desolder the leads easily.
         Bigger joints with lots of solder can overwhelm solder wick, saturating it before
    much solder is removed. To clean out an entire large joint might require a foot of wick,
    which isn’t cheap. These are jobs for solder suckers. After applying the sucker a few
    times, you should be left with only a coating of solder on the joint. A sucker will not
    remove that, so finish up with wick.
         As mentioned in Chapter 6, avoid using a spring-loaded solder sucker on
    static-sensitive components like CMOS chips and MOSFET transistors. The rapid
    release of the plunger can generate static charges capable of damaging those parts.



Surface-Mount Components
    Wicking surface-mount parts is easier because all of the solder is touching the wick,
    and many of the lands are very small and readily heated. Most surface-mount pads
    will desolder without incident. If the solder on a small land won’t flow into the wick,
    try the same trick I described above: add some fresh solder to the joints before trying
    to desolder them.
214   How to Diagnose and Fix Everything Electronic

            Large lands on power supply and ground buses may still be hard to heat, but
      a normal iron will take care of most of them. Using a big gun on a surface-mount
      component is asking to destroy the part and quite possibly the board. Tiny SMT
      (surface-mount technology) resistors and capacitors have sputtered-on solder pads.
      Too much heat can delaminate them, making reconnection to the parts’ bodies
      impossible to achieve.
            Most SMT components are glued in place before being machine-soldered at the
      factory. Very often, desoldering the ends of a part will break the glue and free the part,
      but not always. If you see a red shellac-like blob around the edges of the component,
      it’s glued on and may not budge after desoldering. To move it, wick both ends and
      then heat one end while pushing on the component’s body with a small screwdriver.
      The tiny part may pop off suddenly and blast away into oblivion if you’re not careful.
      Somewhere in the universe there must be a room full of sad, homeless SMT parts that
      flew off circuit boards, never to be found. Plenty of ’em came from my workbench.



Choosing Components
      Any time you need a new part, you just breeze on down to your local electronics supply
      store, buy the exact replacement and pop it in. Um, right, sure you do. Ah, if only real
      life could be like that! We don’t even have local parts stores anymore. And while lots of
      standardized components are available via mail-order, many newer consumer electronics
      products aren’t made from them. Instead, they’re stuffed with all kinds of obscure and
      specialized components nobody but the manufacturer can provide. Luckily, in most
      cases you have a few options.


      Ye Olde Junque Box
      If you’ve stockpiled components, see if what you have is a close enough match. When
      using parts that have been sitting around for a long time, take some fine sandpaper
      to the leads to remove oxidation that will have built up. Otherwise, soldering to those
      leads will be unsuccessful.


      Parts Machines
      There’s a reason I’ve encouraged you to save boards from dead machines. Those from
      the same manufacturer as the unit you’re repairing might use the same component,
      even if they’re a different model. Manufacturers save costs by reusing parts of their
      designs and techniques in lots of models. If you can’t find an exact replacement, you
      still might locate something close enough to work. Check all your parts machines,
      even those made by other companies. You’re more likely to find a compatible part
      from the same type of machine, since the function is similar. So, if you need a part
      for a camcorder, check boards from those; you probably won’t find what you need in
      a DVD player. If you locate something you can use, but the leads are too short, solder
      on a little wire to extend them.
Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 215

   Substitutes
   Substituting a part with something close but not an exact match requires consideration
   of how the part is being used, what parameters are critical, and what you can get away
   with in a particular application. The general idea is that a part with better specs can
   sub for a lesser one, but not the other way around. Even then, there are exceptions.
   Different component types have varying requirements. Let’s look at some common
   ones.

   Capacitors
   Most of the capacitors you’ll replace are electrolytics. Tantalum caps fail pretty often
   too, but they aren’t used much anymore, so you may never run across one. The
   major factors in an electrolytic are its size, its capacitance, its voltage rating and its
   temperature rating. Also, switching power supplies and computer motherboards
   often require caps with especially low ESR, to smooth out the fast, sharp pulses
   those circuits produce. Replacing such parts with standard electrolytics will cause
   malfunction.
        The most important consideration after size—it does, after all, have to fit on the
   board in the allotted space—is voltage rating. Electrolytics simply won’t stand voltages
   higher than their ratings, at least not for long; they fail catastrophically by shorting.
   Their life is reduced even by running them at voltages under but close to their ratings,
   yet some manufacturers will use a 15-volt part at 13 volts, leading to frequent failures.
   In cases like that, a replacement with a higher voltage rating than the original part is
   not only okay, it’s desirable.
        The capacitance rating is not as critical as you might suppose. Most ’lytics have
   rather wide tolerances, in the range of -20 to +80 percent. If the cap is being used
   to couple signals from one stage to another, the capacitance value is more important
   than it is when the part is a bypass or filtering cap. You might find a few electrolytic
   coupling caps in audio and video gear. In audio amplifier stages that use caps of a few
   microfarads from an emitter to ground, it pays to keep the value close to the original,
   because a higher value might increase low-frequency response, upsetting the audio
   quality.
        You will see tons of electrolytics in power supplies and for bypassing and
   filtering in all kinds of products, from simple analog devices to today’s most complex
   digital gear. Those are the parts that usually need replacement. If your available
   replacement’s value is no more than 50 percent higher, go ahead and use it. A little
   extra filtering never hurt anything, and +50 percent is likely within the stated
   tolerance of the original part anyway. To be sure the new part isn’t at its maximum
   tolerance value of, say, 80 percent over the stated value, measure it with a capacitance
   meter. Despite their wide stated tolerances, most electrolytics I’ve measured have
   been within ±20 percent or so of their printed values.
        Combining capacitors to get near the needed value is fine in most applications.
   I don’t recommend it, though, for the big storage cap at the input of a switching power
   supply (near the chopper), or in other high-voltage circuits. Putting caps in parallel
216   How to Diagnose and Fix Everything Electronic

      adds their values, and putting them in series drops the final value according to the
      following formula:

                                              1
                                         1   1   1 ...
                                           +   +
                                         C1 C2 C3

      Capacitors combine exactly opposite to how resistors do. For more info, see the
      section “Resistors” a bit later in the chapter.
           When putting polarized capacitors in series, be sure they connect + to –, so you
      wind up with one + and one – at the ends of the string. When you parallel them,
      connect all the + terminals to each other and all the – terminals to each other. In
      either case, be sure each capacitor’s voltage rating is equal to the entire applied
      voltage. When in series, the individual caps won’t really be subjected to the full
      voltage during normal operation, but a big voltage spike can occur when power is first
      applied, so it’s a smart safety move to be certain every one of them can handle it.
           Especially in power supply applications, the cap’s temperature rating matters.
      Electrolytics that get charged and discharged very fast, as they do in a switcher, can
      become plenty warm from the power dissipation of their internal resistance. Standard
      ’lytics are rated to operate at 85° C, with higher-temperature caps rated as high as 150° C.
      Manufacturers hate paying for things they don’t need, so respect the temperature ratings
      if you want the repair to last. For quick testing purposes while troubleshooting, you
      can disregard the ratings because the part won’t be running long enough to fail from
      overheating.
           Tantalum capacitors should always be replaced with the same type. They have
      lower impedance at high frequencies than do standard electrolytics, and are used only
      where that matters. Replacing a tantalum with a garden-variety electrolytic will result
      in performance degradation or circuit failure. The capacitance tolerance of tantalums
      is much tighter than that of standard electrolytics, so use a part with the same value.
      An increased voltage rating is fine, however.

      Diodes
      Diodes and rectifiers have four primary characteristics: forward voltage, reverse
      voltage, current and speed.
          The forward voltage spec tells you how much voltage can be across the part in its
      conducting direction. You won’t often see this specified, because in an AC circuit the
      forward and reverse voltages are usually the same. The reverse voltage is specified
      as PIV, for peak inverse voltage, and it tells you how much voltage the diode can
      withstand in its nonconducting direction. Exceed the PIV, and the part will arc over
      inside and be destroyed.
          The current rating indicates how much current can pass in the conducting
      direction without overheating the part and burning it out. No current should flow in
      the reverse direction, of course.
          The speed of a diode is very important in some small-signal applications like radio
      signal detection. It’s also critical on the low-voltage side of a switching power supply,
Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 217

   where the part will be rectifying the fast pulses from the conversion transformer.
   In the sections of power supplies operating at the low frequency of the AC line, any
   rectifier is more than fast enough. The bridge or individual rectifiers at the AC cord
   side of a switcher are not high-speed devices; nor are the rectifiers on the low-voltage
   side of a linear supply.
        When subbing a normal, low-speed rectifier or bridge, pay attention to the PIV
   and the current rating. As long as those are equal to or higher than the original part’s
   ratings, the new part should work fine.
        Look up high-speed rectifiers in a substitution book or online. Replace them with
   parts of equal or better PIV, current and speed. Never replace a high-speed rectifier
   with a low-speed part, even if the PIV and current specs are fine. It simply won’t
   work.
        Some products use lots of glass small-signal diodes. Look for numbers like 1N914
   and 1N4148. They’re interchangeable. Even if you see no number on the diode, either
   of those numbers should work fine. Just be sure the diode you’re replacing is in fact
   a simple diode and not a zener. There are some other special-purpose diodes, too,
   including germanium diodes (also glass but noticeably larger than normal silicon
   diodes), gallium-arsenide diodes, tunnel diodes, and varactors. They’re found in
   receiver front ends and other weak-signal, exotic applications. You won’t run into
   them very often, but they must be replaced with diodes of the same types.

   Resistors
   Many resistors are carbon composition types and easy to sub. What matter most are
   the resistance value and the power dissipation capability. It’s fine to use a 1 percent
   precision resistor in place of a standard 5 percent one, and it doesn’t hurt if the
   replacement is rated to handle more power.
         If the original resistor was a special type, such as a wire-wound or low-noise part,
   it’s important to replace it with the same type. Those kinds of parts are used only in
   special applications. You won’t find them very often in consumer electronics gear,
   but they show up now and then in switching power supplies, preamps and stages
   handling particularly small signals, like receiver front ends.
         If you can’t find the exact value you need, consider the original part’s tolerance
   (see Chapter 7), and try to combine a few other resistors to get to a value well within
   the original part’s specs. For instance, if you need a 3.3 KΩ resistor, you could put a
   2.2 KΩ and a 1 KΩ in series. Resistor values in series add together, so that’d get you
   to 3.2 KΩ. If the original resistor had a 5 percent tolerance, as most do, it could vary
   by ±165 ohms and still be okay. So, 3.2 KΩ would be fine as long as the combined
   resistors’ own tolerances didn’t push their total value outside the tolerance range of
   the original part. Check the real value of the combination with your DMM to be sure.
         Resistors in parallel combine opposite to how capacitors do. The resistance value
   goes down according to the formula shown here:

                                          1
                                     1   1   1 ...
                                       +   +
                                     R1 R2 R3
218   How to Diagnose and Fix Everything Electronic

      Two resistors of the same value will produce half the resistance. The larger the
      resistance of the second resistor, compared to the first, the less effect it has on it.
      Play around with a few resistors by combining them in parallel and measuring them,
      and you’ll get the hang of it.

      Transistors
      Transistors are the most complicated parts to substitute. Major semiconductor
      manufacturers used to give away large transistor substitution books filled with hundreds
      of pages of transistor types and their brands’ appropriate cross-referenced substitute part
      numbers. Because many types of transistors have similar characteristics, a few hundred
      parts can sub for thousands of parts.
           These days, you can look up this stuff online, but you may run into numbers
      for which you can’t find a cross, or there might be a valid sub but you can’t get one.
      Alas, some parts are still made of unobtainium. Even when a substitute component is
      available, you may prefer to speed up the repair process by using a part you already
      have.
           To choose your own substitute requires some understanding of the part’s application
      and how a change in characteristics would affect circuit performance. Some functions,
      like simple switching of voltage to direct it to various circuit stages or turn an indicator
      on and off, will work with just about any transistor of the same basic construction
      (bipolar or FET) and polarity. Others, such as high-frequency signal processing or
      current amplification in complementary output stages, often require stringent adherence
      to the original part’s specs.
           All this assumes you know the old part’s number. Usually you will, but at times you
      might have to fly blind. If the original transistor literally blew apart, which occasionally
      happens when a heck of a lot of current has been pulled through one, there may not
      be a number to read! I’ve seen SMT output transistors in LCD backlight inverters blow
      so hard that there was little left between the solder pads. Even when the number is
      visible, it could be a proprietary house number, with no cross-reference to a sub. And
      some transistors, especially tiny SMTs, show no numbers in the first place.
           If you’re lucky, the board will be marked with ECB or GDS, showing what terminal
      goes to which pad. ECB indicates emitter, collector and base, thus a bipolar transistor.
      GDS means gate, drain and source, the terminals of a FET. Those markings also give
      you strong clues to the part’s polarity. If C goes to the positive side of things, it’s an NPN.
      If E does, it’s PNP. With a FET, if D is positive, it’s an N-channel part. If S is positive, it’s
      P-channel.
           Without board markings or a part number, the transistor is a total mystery. Use
      your scope and understanding of basic transistor operation to deduce the part’s
      polarity and layout of connections. Start by looking for the power supply voltage
      feeding it. If it’s positive and fed through a resistor or a transformer, you’ve probably
      found the collector of an NPN transistor or the drain of an N-channel FET. Find the
      stage’s input by looking for whatever signal operates the transistor. If it’s a continuous
      signal, you should see it. If it’s something that happens only when you press a switch
      or some other operation signals that area of the circuit, create those conditions and
      find the signal. When you find it, you’ve found the base or the gate. Whatever’s left
      will be the emitter or the source.
Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 219

        Most bipolar transistors are NPN. If the connection to the positive supply line is
   direct, without a resistor, or there is a resistor but it’s of very low value, the transistor
   could be PNP, and that connection would be its emitter. Find what looks like the base
   by scoping for signals. See if there’s a resistor from the base to the transistor terminal
   closest to the supply. PNPs are used to turn on and pass current from the supply to
   some other circuit when the input signal goes low, toward ground. The resistor going
   up toward the supply keeps the base high and the transistor turned off until the input
   signal pulls it low. You’ll find PNP circuits of that sort in power switching sections of
   battery-operated products.
        Assume the part is an NPN bipolar transistor, and you’ll be right most of the time.
   If your replacement turns out to be the wrong polarity, the circuit won’t work, but it
   shouldn’t do any damage.
        All bets are off if the transistor is part of a complementary push-pull amplifier.
   They use NPNs and PNPs in more complicated, hard-to-deduce ways. And if the
   original part was a FET, the issues of enhancement and depletion mode, and
   JFET versus MOSFET, make the whole thing very tough to fathom. Getting the
   identification correct requires your understanding how those parts work and
   looking at the bias on the gate terminal to infer what the output should do as the
   input changes.
        Don’t try to sub chopper transistors in switching power supplies without knowing
   the correct part number and finding a legitimate sub from a cross-reference. Most
   choppers are power MOSFETs with specs that must be closely matched for reliable
   operation. Even if a sort-of-close sub works, it probably won’t run for long before
   failing. Sometimes even a legit sub will die in a hurry, and the only part that will work
   is an exact replacement of the original part number. The same is true of horizontal
   output transistors in CRT TVs, another application involving fast pulses at fairly high
   voltages and currents.
        In some cases, the original and replacement transistors are electrically compatible
   but their arrangement of leads, called pin basing, is different. Most small-signal
   American transistors are EBC, left to right, while Japanese parts are usually ECB.
   You can replace one layout with the other as long as you switch the two leads, being
   careful not to let them touch as they rise from the board toward the transistor. Small
   FETs are usually SGD. Power transistors are usually BCE, with C connected to the
   metal tab (if there is one), but check to make sure. Power FETs typically use GDS,
   with D connected to the tab.
        Once you’ve figured out what should go where, whether from the original part
   or from scoping and deducing, you can proceed with trying out a new part. The
   primary characteristics to be concerned with are gain, high-frequency cutoff point
   and, with larger parts, power dissipation capability. Secondary characteristics, but still
   very important, are the maximum voltages permitted from base to emitter and from
   collector to emitter.
        Very often you’ll find a transistor that’s pretty close but has a little more or less
   gain. Depending on the application, that might work. If the circuit is linear, producing
   output proportional to the input, the transistor isn’t normally saturated (fully turned
   on), so a slight gain difference may not cause a problem. In switching circuits like
220   How to Diagnose and Fix Everything Electronic

      backlight inverters, though, inadequate gain can result in lots of heat from the
      transistor’s not-fully-turned-on resistance, burning out the part in a hurry. Too much
      gain in a linear circuit may cause distortion, increased output or spurious signals. Not
      enough usually just results in a bit less output.
           The high-frequency cutoff point specifies at what frequency the transistor’s gain
      will have decreased to one. In other words, it won’t be amplifying at or above that
      frequency. In low-frequency applications such as audio, any transistor will be more
      than fast enough. At radio frequencies, the situation can be quite different, requiring
      a transistor whose cutoff frequency is approximately equal to the original part’s spec.
      Too little might result in low or no output, while too much could result in unwanted
      harmonics or spurious signals riding on the desired one. When in doubt, go for too
      much, as long as the difference isn’t excessive; at least the thing will try to work.
           Power dissipation is very important. The new part should be able to dissipate at
      least as much power as the old one. A better dissipation spec is fine.
           Maximum permissible inter-electrode voltages must be respected. Exceed them
      and the transistor might emit some of that magic smoke. Most transistors’ collector-
      to-emitter specs are well beyond what a small-signal circuit produces. The circuit’s
      base-to-emitter voltage, however, could exceed the capabilities of some replacement
      parts, so keep an eye on that. Large parts used in output stages can have pretty high
      voltages applied from collector to emitter, so don’t take that spec for granted.
           If all this seems overwhelming, stick to replacement part numbers from a cross-
      reference book or online source, and you’ll be fine. Even with expertise, matching up
      transistors is very much a roll of the dice. See, I told you it could get complicated!

      Zeners
      The purpose of a zener diode is to break down nondestructively in the reverse direction
      and conduct when the part’s reverse voltage spec, or zener voltage, is reached. The
      important specs are the zener voltage and the power dissipation. Unlike normal diodes,
      zeners’ dissipation limits are specified in watts, not amps. Always replace a zener with
      one of the same zener voltage and at least as much dissipation capability. A higher
      dissipation spec is fine.
           You can put zeners in series to add their voltages, but don’t parallel them to
      increase dissipation capability; even zeners with the same zener voltage won’t start
      conducting at exactly the same voltage, so one will always take more current than the
      other, resulting in its premature failure. When combining them in series, be sure that
      the wattage of each zener is at least as high as the original part’s rating, and watch the
      polarity. Each zener should feed the next one cathode to anode, so you wind up with
      one anode and one cathode at the ends of the string.



Installing the New Parts
      Once you’ve procured or substituted components, it’s time to put them in! Proper
      installation is crucial for successful, long-term repair. Let’s look at some issues specific
      to various kinds of parts.
Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 221

   Through-Hole
   Replacing a through-hole component is pretty easy, requiring nothing more than
   pushing the leads through the holes, bending the ends a little so the part doesn’t fall
   out, soldering the leads and then clipping off the excess.
       If the part is attached to a heatsink, it’s a little more complicated, but not much.
   For a free-floating heatsink bolted or clamped to the top of the component, install
   the heatsink before soldering the part to the board. When the part mounts on a fixed
   heatsink, put the leads through the board’s holes without soldering them, and then
   screw or clip the component to the heatsink. Solder the leads only after the mounting
   procedure is complete.
       If the original part used heatsink grease, you need to do the same with the new
   one. The grease used is a special silicone compound formulated for maximum heat
   transfer. You can get it from online parts houses, and computer supply shops that
   carry CPU upgrades and bare motherboards also carry it. Most heatsinks, including
   those with mica or thin plastic insulators, do require the grease. Those with rubber
   separators usually don’t, though. Figure 12-3 shows typical insulator setups requiring
   thermal grease.
       A thin smear of the special grease on one of the mating surfaces helps heat
   transfer across the less-than-perfect contact area, filling in tiny gaps and increasing
   effective surface area. Too much grease can separate and insulate the surfaces,
   reducing heat flow, so don’t overdo it. Smear on the grease with a swab, and be
   careful not to put bending pressure on the insulator or it may break. Mica insulators
   are especially brittle, and even a single crack can lead to a short later on. To avoid
   bending it, place the insulator on your workbench before applying the grease.
       The insulator’s job is to isolate electrical contact between the component and
   the heatsink while facilitating heat transfer. If there’s no insulator, either the part




              Figure 12-3     Transistor mounting hardware with screw sleeves
222   How to Diagnose and Fix Everything Electronic

      has no contact point on its case, as with an all-plastic transistor, or it’s okay for it
      to be connected to the heatsink. Voltage regulators sometimes have their ground
      connections on the metal tab, so contact with a grounded heatsink is a good thing.
      Many power transistors, though, have their collectors or drains at the tab. Those
      are usually connected to voltage sources, and contact with ground would be a short.
      Insulators are used to avoid the connection.
           When there is an insulator, and the component has a metal tab, the mounting
      screw will pass through a plastic washer with a sleeve. Be certain to use it, and
      watch its orientation. The sleeve should fit into the hole on the transistor’s tab,
      preventing the screw from touching the inside of the hole.
           Tighten the mounting screw more than you would a screw holding a board down
      or a case together. You want good heat transfer, and that takes some pressure. Don’t
      overdo it to the point of breaking the insulator or stripping the screw, of course.
           Occasionally, you will find a thermistor (a heat-sensitive resistor) glued to the
      case of a power transistor, especially in the output stage of a push-pull audio amplifier.
      Thermistors are used to adjust the bias of bipolar power transistors as the parts heat
      up, because their gain and optimum bias point drift with temperature. If you can get
      the thermistor off without destroying it, glue it with epoxy to the new part. If you
      can’t remove it, you’ll need a new thermistor. Look up its part number and order one
      just like it.


      SMT
      Putting in a new SMT part is a bit tougher than installing a through-hole component,
      thanks to the size scale. How do you hold it in place long enough for soldering? Gluing
      is not recommended. Sure, the manufacturers do it, but they have special glue made
      for the purpose, and we don’t. More than likely, some other line runs underneath the
      component, and a later attempt to remove the glue will tear the copper off the board.
      Also, the electrical properties of the glue you might use are a wildcard; you have no
      idea how its presence might affect circuit performance. It could exhibit capacitance or
      even conduct current.
           To get an SMT in place, first use wick to clean the board’s solder pads so that
      there are no raised bumps of solder on them. You want the SMT to lie flat. Melt a little
      solder onto your iron’s tip. Now place the part on the board and line it up carefully
      with a tiny screwdriver. Center it between the pads so it can’t create a short across two
      lands. Hold down the body of the SMT with the screwdriver while touching the iron’s
      tip to one end of the part. The solder on the tip should flow onto the board, making a
      joint at that end. Don’t worry about getting a good joint; all you want to do is prevent
      the part from moving.
           Once the component is held in place by the solder on one end, solder the other
      end properly. Then go back and redo the messy end. Take a good, close look with
      your magnifier to be sure you haven’t created any solder bridges to adjacent pads or
      parts.
 Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 223


Finding Parts
    Proprietary components have to be procured from the manufacturer (unlikely these
    days, but worth a try), the component maker who supplied them (possible) or a parts
    unit. For popular gadgets, finding a parts unit may be the easiest way to go. Check eBay.
        Standard components are widely available through online mail-order, but many
    parts houses have minimums, so you might have to spend a lot more than the part
    is worth. Oh well, you can always stock your components supply with other goodies
    you might use later. Or, you can save up your parts needs until you have a big enough
    order. That’ll delay your repair work a long time, though.
        Here are some places to look for components:

     •	 RadioShack (radioshack.com) This seller’s parts variety is small, but the
        company offers a few transistors and chips, along with standard 5 percent resistor
        values and some electrolytics.
     •	 DigiKey (digikey.com) This mail-order parts house has just about everything you
        could ever want. Its catalog is overwhelming, and you can download it as a PDF.
     •	 Mouser Electronics (mouser.com) Another powerhouse, Mouser has a wide
        variety of components, including many used in consumer electronics devices.
     •	 All Electronics (allelectronics.com) This is a surplus house with lots of
        interesting material at bargain prices. It has inexpensive, generic backlight inverter
        boards that can be retrofitted to LCD monitors, though the boards lack terminals for
        brightness control.

        Do an online search and you’ll turn up dozens more sources for both prime and
    surplus components.



Saving Damaged Boards
    When you desolder a through-hole component, one unfortunate result of failing to get
    the hole hot enough is that its copper lining comes out with the lead. If you see what
    looks like a sleeve around the lead, you’ve torn out the copper. On a double-sided
    board, it’s not a catastrophe. When you replace the part, be sure to solder both the top
    and bottom contact points, and all will be well. You might have to scrape some of the
    green solder mask coating off the top area to get contact between the lead and the foil.
    That’s best done with the tip of an X-Acto knife.
        Pulling the sleeve out of a multilayer board can destroy it because you have no
    way to reconnect with interior foil layers that were in contact with the sleeve. If you’re
    lucky, that particular hole might not have had inner contacts, and soldering to the top
    and bottom may save the day, so it’s worth a try. Don’t be surprised, though, if the
    circuit no longer works.
        If you can figure out where they go, broken connections can be jumped with wire.
    On double-sided boards, it’s not too hard to trace the lines visually, though you may
    have to flip the board over a few times as you follow the path. When you find where
224   How to Diagnose and Fix Everything Electronic

      a broken trace went, verify continuity with your DMM, from the end back to the break.
      Don’t forget to scrape off the solder mask where you want to contact the broken line.
            Wire jumping can help save boards with bad conductive glue interconnects, too.
      On a double-sided board, you can scrape out the glue and run a strand of bare wire
      through the hole, soldering it to either side. Forget about trying this on a multilayer
      board, however; you’ll probably trash it while trying to clean the hole. On those, it’s
      best to run an insulated wire around the board from one side to the other. That adds
      extra length to the conductive path, which could cause problems in some critical
      circuits, especially those operating at high frequencies. At audio frequencies, it should
      be fine. If some interior layers are no longer making contact with the glue, this won’t
      work. Most conductive glue boards I’ve seen have been double-sided, making them
      suitable for wire jumping.
            If the board is cracked from, say, having taken a fall, scrape the ends of the copper
      lines at the crack. It’s possible to simply solder over them, bridging the crack, but that
      technique tends to be less permanent than placing very fine wire over the break and
      soldering on either side. To get wire fine enough, look through your stash of parts
      machines for some small-gauge stranded wire. Skin it, untwist it and remove a single
      strand.
            Sometimes there are multiple broken lines too close to each other for soldering
      without creating shorts between them. To save boards like that, scrape the solder
      mask off close to the crack on every other line. Then scrape the in-between lines
      farther away from the crack. Use the bare wire strands to fix the close set, and use
      wire-wrap wire (very thin, single-strand, insulated wire used with wire-wrap guns
      for prototyping experimental circuitry) or enamel-insulated “magnet wire” to jump
      the farther set. Wire-wrap wire is especially good for this kind of work because its
      insulation doesn’t melt very easily, so it won’t crawl up the wire when you solder
      close to it, exposing bare wire that could short to the repaired lines nearby. Plus,
      it’s thin enough to fit in pretty small spaces. For even tighter environments, use the
      magnet wire. Just be sure to tin the ends of the wire to remove the enamel, so you’ll
      get a good connection.
            It’s possible to repair broken ribbon cables in stationary applications (the ribbon
      doesn’t move or flex), if they are the copper-conductor type of ribbons, not the very
      thin, printed style. Fixing cracks with wire is a tedious, time-consuming technique,
      but it works. Accomplishing it without causing shorts takes practice and isn’t always
      possible with very small, dense boards and ribbons.
            On multilayer boards, cracks and torn sleeves are extremely difficult to bypass. If
      you have a schematic, you may be able to find the path and jump with wire. Without
      one, it’s pretty much impossible when the tracks are inside the board.



LSI and Other Dirty Words
      Back in Chapter 7, I promised to describe a trick for resoldering big ICs with very
      close lead spacing. Those large-scale integrated (LSI) chips with 100 leads are in just
      about everything these days. It’s not likely you could find a replacement chip, so why
      would you want to resolder one?
Chapter 12    Presto Change-O: Circuit Boards and Replacing Components 225

        With so many leads, an intermittent connection to an LSI is not uncommon. SMT
   boards are factory-assembled with reflow soldering, in which solder is applied to the
   pads and then reflowed onto the component leads with hot air, infrared lamps or in a
   special oven. Reflow soldering relies on low-temperature solder that can break after
   the numerous heating and cooling cycles encountered in a product’s normal use. Now
   and then one connection out of an LSI’s long row of them will go flaky. The leads are
   so close together that there’s no way to apply solder to one without causing a short to
   the adjacent leads.
        Here’s the trick: go ahead and short them! With all power removed, of course,
   solder away and let as many leads get shorted together as you want. Once you have
   good solder on the problem lead, lay solder wick across the leads where the excess
   solder is shorting them. Heat it up and wick off the excess, but don’t wait until the
   leads are bone-dry. Pull the wick off a little sooner. If you get the timing right, you’ll
   be left with a perfectly soldered row, with no shorts. The wick soaks up the solder in
   between leads faster than what’s underneath them (where you want it to stay).
        If you wait too long and wind up removing so much solder that the connections
   to the board aren’t solid anymore, resolder the area and do the wick trick again. After
   you try this procedure a few times, you’ll get the hang of how long to wait before
   pulling the wick. I’ve had tremendous success with this approach. The one caveat
   is that it’s hard to wick out solder if it gets under the edges of the chip. To avoid
   that problem, solder as far from the body of the IC as possible. That helps prevent
   overheating the chip, too.
        When you’re all done, use your magnifier to verify that the contact points with
   the board are soldered, and that no bridges exist between leads. Honest, it really does
   work! I’ve even replaced a few LSIs this way, using chips from parts units. Getting
   those babies lined up accurately on all four sides…well, that’s another story.
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Chapter                    13
   That’s a Wrap:
   Reverse-Order Reassembly

   Y    ou fixed it. Congratulations! Now it’s time to put everything back together. Just
        screw the boards down, plug in the connectors, close the lid and you’re done,
   right? Well, sometimes, but not performing the reassembly methodically can lead to
   all kinds of trouble, from failure of your repair to new damage, and even to danger
   for the product’s user. You’re sewing up your patient after the operation, and it’s
   important to put in the stitches carefully and avoid leaving a scar.



Common Errors
   It might seem absurd to think that one could reassemble a machine and have parts
   left over, but it happens all the time. You snap that final case part into place, breathe
   a sigh of relief, glance at the back of your workbench, and there it is: some widget
   you know belongs inside the unit, but you forgot all about it. Hmm, is it really worth
   all the trouble to backtrack, just for one little, seemingly non-essential item? Sigh….
   Time to pull the whole mess apart again. It’s very easy to forget to replace a bracket, a
   washer, a shield, a cover or even a cable. The unit might function without one of those
   pieces, but it’s not going to work completely right.
        Putting the wrong screws in the wrong places may have no consequences, but
   it also could seriously damage the device. If a screw that’s too long presses against
   a circuit board, it might short whatever it touches to the chassis, hence to ground.
   You flip on the power and, voilà, you have a new repair job on your hands. See the
   definition of magic smoke in the Glossary.
        Overtightening screws can strip their heads, making it very hard to get them
   out again. It can also break plastic assemblies and cause cracks in the case.
   Undertightening screws may lead to their falling out later, possibly jamming
   mechanisms or shorting out circuitry if they’re internal screws.



                                                                                      227
228   How to Diagnose and Fix Everything Electronic

          Our memories can really fool us sometimes. You’re certain that part went over
      here, but now it doesn’t quite fit. So, you press a little harder, because you know that’s
      where it goes. Snap! Oh, right, it went over there, not over here. If something doesn’t
      want to fit together, it probably belongs somewhere else. This kind of error happens
      more with mechanical parts than with electronic components.
          Manufacturers try to make internal connectors different enough from each other
      that only the correct cables will fit. In products with lots of connectors, though, there
      may be ambiguity. CRT projection TVs, for instance, have so many connectors that
      there’s just no way to make them all distinct, and sometimes a color code is all you
      have to guide your reassembly. It’s not hard to get one wrong. Guess what happens
      when you turn the set on.
          Even when you put ribbon cables in the right connectors, it’s easy to damage
      them. They’re delicate and easily torn or folded hard enough to snap their printed
      copper conductors. The connectors are fairly breakable too, especially the tiny ones
      in pocket-sized products. Those plastic sliders that hold the cables in place snap off
      without much pressure. Worse, the entire connector can break its solder joints and fall
      right off the board, just from the stress of being pushed on while the cable is inserted
      and clamped down. I’ve seen that happen on digital cameras and pocket video gear.



Getting Started
      To begin reassembly, reverse the order in which you took the machine apart. When
      the unit has multiple boards, you’ll need to get the inner ones reinstalled first. If a
      board that’ll wind up under another board has connectors, put the cables in before
      covering up that board.
          In older gear, take a look at the ground lands on circuit boards, where the screw or
      the metal bracket makes contact. They exist to connect those points to ground via the
      chassis, and a poor connection due to oxidation or corrosion can seriously affect the
      product’s performance. Clean them up with some contact spray or, in extreme cases,
      fine sandpaper. Make sure the screws are tight, so contact will be reliable, but don’t
      overtighten to the point that you might crack the board. Heating and cooling in larger
      items, and physical stress in portable devices from being bounced around, can cause
      cracks later on if the screws are extremely tight.
          A little sealant on a screw head is better than pushing the limits of tightness.
      Manufacturers and pro shops use a type of paint called glyptal to keep screws from
      loosening. Swabbed around the edges of the screw head, it is highly effective. You can
      use nail polish. Don’t glob it on; just a little smear will do fine. Be careful not to cover
      components or their leads, and let it dry before closing the case, so the outgassing
      won’t remain inside. I use red polish so I can see where I’ve been, should I open the
      unit again later on.
          Placement of wires and cables is called lead dress, and it can be surprisingly
      important. When you took the unit apart, you may have noticed that some wires were
      tacked down with hot-melt glue or silicone sealer. If the manufacturer went to the
      trouble to do this, there was a reason. Maybe the wire needed to be kept away from
                   Chapter 13       That’s a Wrap: Reverse-Order Reassembly 229

   a hot heatsink that could melt its insulation. Perhaps a cable carries a weak, delicate
   signal that would receive interference if it got too close to some other element of
   the machine. This can be the case with the cables going to video head drums. Or
   maybe the reverse is true: the cable would cause interference to other sensitive
   circuits. Wires carrying high voltages, like those used to run projector lamps and
   LCD backlights, may need to be kept away from all other circuitry to avoid not only
   interference but the possibility of arcing. The closer a high-voltage wire is to ground,
   the more those devious electrons want to punch through the insulation and get there.
   Give ’em time and they just might.
         If the manufacturer tacked wiring down, put it back the way you found it. Hot-
   melt glue is somewhat flammable, and it melts with heat, of course, so it isn’t used
   much in larger products. Now and then, you may find it in smaller items that don’t
   carry much voltage or produce significant heat. To tack wires back down into it, you
   can melt the blob with your plastic-melting iron, avoiding any other wires, or drip a
   little more glue on top from your glue gun.
         More often, you’ll find silicone sealer used to secure wires. The type used is
   called RTV, and it’s best to replace it with the same kind, because it offers the correct
   insulating strength. RTV is available at most hardware stores, and electronics supply
   houses carry it too.
         Now and then, and especially in small-signal RF stages operating at very high
   frequencies, you’ll see blobs of wax covering transformers and capacitors. The wax
   holds the parts to the board and dampens vibrations that can cause noises in signals or
   frequency instability in oscillators. In some circuits, the capacitance of the wax may
   affect circuit operation, so it’s best to remelt and reuse the original wax. It’s not the
   same stuff that’s in the candles on your dinner table.
         Even if the manufacturer took no extra care with wiring, you should pay attention
   to the issue to achieve maximum product reliability and safety. Could a power supply
   lead touch a heatsink? Is the cable from a tape head going right by the power supply?
   And, perhaps most important, is any wire or cable placed such that it’ll get crimped
   by a circuit board or part of the case when reassembly is complete? The sharp ends
   of a board’s component leads can go right through a wire’s insulation, with disastrous
   results. Crimping caused by the case can break the wire or cut through the insulation
   and short it to the chassis. With a wire carrying unisolated AC power, a crimp could
   even present a shock hazard. If the case halves don’t mate properly, a wire is probably
   in the way. Don’t just squish them together and go on.
         These scenarios may sound farfetched, but they’re really pretty common. I always
   make it a point to watch the wiring as I close up the case, imagining where things will
   be and how they’ll press on each other before I actually snap the halves together or
   tighten the final screws.



Reconnecting Ribbons
   Insert ribbon cables into their sockets carefully. It isn’t hard to put them in wrong,
   which can lead to anything from no operation to circuit damage. Most ribbons have
230   How to Diagnose and Fix Everything Electronic

      bare conductor fingers only on one side. If you get one of those in upside-down, the
      product won’t work, but it’s unlikely to cause damage unless the socket has U-shaped
      contacts that touch both sides of the ribbon. There are some like that.
            Double-sided ribbons offer more opportunity for calamity. How do you know
      when they’re in the right way? Many are keyed with a notch at one end so they can’t
      be inserted upside-down. Some are not, though. I sure hope you heeded my advice in
      Chapter 9 to mark the darned things! If not, see if the cable has a bend or curvature
      suggesting its original orientation.
            As discussed in Chapter 9, some ribbon connectors have no latches, and the
      ribbon just slides in. That type requires some force for proper insertion. With such
      a connection style, the ribbon cable will have a stiff reinforcement layer at the end.
      Even when you find one, take a good look at the connector to be sure it has no latch
      that slides in or flips up, because some latch-type connectors accept reinforced
      ribbons too. If you see no latch, grasp the cable’s reinforced tab, carefully line up the
      ribbon with the connector and press it in firmly.
            Connectors with latches are easier to manipulate. First, be sure the latch is open.
      Slide it out or flip it up. The flip-up kind will stay up while you insert the ribbon,
      but the slide style has an annoying habit of going partway in before you want it to,
      blocking full reinsertion. One end may slide in, resulting in a crooked latch. If that
      happens, pull that end back out, remove the ribbon and try again. You might have to
      hold the slider’s ends with one hand while you insert the ribbon with the other.
            Latch-style connectors require almost no insertion force. Gently slide in the
      ribbon until it stops. Don’t press firmly here. Look down at the top of the connector
      and verify that the ribbon isn’t crooked. Then close the latch while holding the ribbon
      in place with your other hand. That’s easy with flip-up latches and a little harder with
      sliders. Occasionally, I’ve had to close sliders one end at a time with a thumbnail or
      a screwdriver while holding the ribbon in my other hand. To avoid a crooked result,
      it’s better to close the ends at the same time, but now and then you gotta do what you
      gotta do.
            When the latch is closed, look again at the exit point and make sure the ribbon
      is straight. You may see the edges of the bare conductors sticking out. That’s fine as
      long as they’re even and you’re sure the ribbon is in all the way. Many of them are
      designed that way.
            Special ribbons used for hard drives and other very dense applications can
      have two sets of fingers, one behind the other. These will always have latch-style
      connectors. Be absolutely certain the ribbon is fully inserted, so there’s no chance the
      wrong set of fingers could make contact with the mating pins in the connector.



Oops!
      If you were unlucky enough to break the latch on a sliding-style connector when you
      removed the ribbon, don’t despair. The object is to get pressure on the conductive
      fingers so they make good connection with the socket. Find some thin, soft plastic
      from, say, the bottom of one of those little pudding cups in which you keep screws.
                     Chapter 13       That’s a Wrap: Reverse-Order Reassembly 231

   Cut the plastic into a rectangle that just fits into the socket and sticks out a few
   millimeters. Trim carefully so the edges line up well with the edges of the socket,
   without a gap. Now put the ribbon in and wedge the plastic piece in to replace the
   broken latch. Be absolutely sure to insert it on the side of the ribbon that does not
   have the conductive fingers, or the plastic will block the connection. I got that wrong
   once and went around in circles for hours trying to figure out why that confounded
   shortwave radio wouldn’t turn on anymore!
        If you get the thickness right, it’ll take a little pressure to slide in the plastic, but
   not a lot. If it slides in very easily, it may not put enough pressure on the ribbon to
   make good connections. The few times I’ve had to do this, I used forceps to push in
   the plastic piece. Needlenose pliers will work as well.
        Flip-up latches are much harder to repair. It might be possible to modify the
   socket by melting a piece of plastic over it and converting it to a sliding arrangement,
   and then using the plastic insert approach, but it’d be a difficult modification to pull
   off, considering the size scale of some of these connectors. Trivial as it may seem, a
   broken flip-up latch often means the end of the product unless you can scrounge a
   latch from a parts unit.



Layers and Cups
   Here’s where those pudding cups come into play. If you used them as I suggested
   back in Chapter 9, your innermost layer’s screws will be in the cup second to the top,
   just under the empty protective cup. Put that layer’s boards, shields and assemblies
   into place and fasten them with those screws. If the screws are of different sizes or
   styles, you should have taken digital photos or made a drawing and placed it in the
   cup with the screws. Be certain to use all the loose parts in the cup, because you won’t
   be able to reach that layer once you reinstall the layers covering it.
        When you’re ready for the next layer, pull the empty cup and put it aside,
   exposing the next set of screws. Continue on with the next layer, and so on. When
   you’re all done, you should have a nice set of empty cups ready for the next project. If
   you have screws left over in the final cup, remember to check under labels and rubber
   feet for hidden holes.



Oh, Snap!
   Those nasty hidden snaps are much easier to close than they were to open! Line
   them up carefully and apply pressure until they pop into place. The edges of the case
   should fit smoothly. If there’s a bulge, either the snap isn’t all the way in or a wire is
   caught underneath.
        Sometimes a case has to be snapped together at one end before the other, even if
   it didn’t come apart that way. Look at the style of snap, and how it fits together should
   be apparent. If you get it wrong, you might break a snap, but it’s not a big deal. Heck,
   they can break even when everything is done right. Often you can live without one
232   How to Diagnose and Fix Everything Electronic

      or two. If a snap’s loss makes the case wobbly, it might be worth some careful repair
      with your plastic-melting iron. Be sure to pop the case apart first; trying to melt plastic
      near the outside will almost certainly result in very visible damage. Even from a
      half-inch away, the iron puts out enough heat to soften and deform many plastics. A
      repaired snap is weak, so you may get only one chance to close up the case properly.
      Still, it’s better than nothing. If you hear something floating around inside the unit
      after you finish putting it together, the snap has broken off. Open the case and remove
      the plastic piece.



Screwing It Up Without Screwing It Up
      As I mentioned, screws should be reinstalled carefully to avoid damaging them or
      the plastic into which they are screwed. Phillips screw heads are especially easy to
      strip, and trying to remove one is mighty frustrating once you do. Insert all the screws
      in a layer, or on the outside of the case, but don’t tighten them down before the last
      one in that layer is in its hole. Sometimes you’ll need to remove one because it’s the
      wrong length or you suddenly realize the black one went here and the silver one over
      there. Or, an internal bracket doesn’t quite line up and you need to open the case
      again before going on. Once you have them all in, it’s time for the final tightening.
      How tight is right? Hold the screwdriver with your fingers, not in your palm. Turn
      the screws just until they stop, and snug them in ever so slightly. That’s it. Don’t twist
      until you can’t twist any farther.



Done!
      If everything fits together well, you should be ready to fire up the unit and consider your
      repair complete. Be sure to bench-test receivers, projectors and other heat-generating
      products if the work you did could possibly make them run too hot. You’ll want to
      bench-test a projector whose fan or ballast circuit you repaired or replaced, or a receiver
      that needed new output transistors. A digital camera or an MP3 player, of course, won’t
      require that extra step. Now go show off your work and bask in the glory of a job well
      done. You’ve earned it!
Chapter                   14
   Aces Up Your Sleeve: Tips and
   Tricks for Specific Products

   A    lthough the principles we’ve covered apply to pretty much all electronic devices,
        various product categories are different enough that they benefit from specific
   troubleshooting techniques. Let’s look at some of the most common gadgets, their
   typical problems, and how to approach their repair.



Switching Power Supplies
   Since you’ll run into issues with switching power supplies in many kinds of machines,
   let’s cover them first, before looking at the products in which they take up residence.


   How They Work
   Switching supplies rectify the incoming AC into DC and then chop it at high frequency,
   pulling current through a transformer’s primary coil with each pulse. Using a high
   frequency allows the energy to be replenished on the other side of the transformer
   much more frequently than with the old linear approach, which was limited to the
   60-Hz line frequency. So, the transformer doesn’t have to convert as much power at
   one time and can be a lot smaller. The approach also keeps the chopper transistor
   either saturated (turned all the way on) or cut off (turned all the way off) most of
   the time, resulting in high efficiency, since it spends almost no time per pulse in its
   midrange (partially turned on), where transistors act like variable resistors, with all
   the attendant heat resistance generates.




                                                                                     233
234   How to Diagnose and Fix Everything Electronic

      What Can Go Wrong
      Controlling those high-frequency pulses, with their fast rise and fall times, is tougher
      and stresses the components a lot more than did low-speed linear circuits. The fast
      pulses with high-voltage peaks punch holes in transistors’ substrates, and the rapid
      charging and discharging of filter capacitors wears those out too. Consequently,
      switchers fail significantly more often than do linear supplies. Nonetheless, very
      few products still use the old technology; switchers are everywhere, from computer
      supplies to little AC adapters and chargers for cameras and cell phones.


      Is It Worth It?
      Unless the transformer is shorted or open, it’s generally worth repairing a switcher,
      especially if it’s in a product you want to get working again. It might be a waste of
      effort on an AC adapter that could be easily replaced. The transformer rarely goes bad,
      though I’ve seen it happen now and then with that ubiquitous cousin of the switching
      supply, the backlight inverter. The high voltage of an inverter’s output sometimes
      breaks down the insulation between the transformer’s windings. In a normal, AC-
      powered switcher used to create low-voltage DC output, that’s an unlikely scenario.
          If multiple semiconductors have blown from a chain reaction feeding voltage
      from one stage to the next, fixing the supply may be more trouble than it’s worth.
      Especially if the pulse-width modulator chip is dead, getting the part can be a hassle.


      The Dangers Within
      To service a switcher, disconnect the power first! Never work on one with power
      applied unless you have an isolation transformer. Even then, don’t work on powered
      switchers until you gain a fair amount of experience. The dangers are real and
      significant. Did you take off your wristwatch and all jewelry? It’s especially important
      now. Be sure to wear shoes, too.
           Switchers are organized in two sides: the primary side, connected to the AC line
      and with no ground connection to the rest of the product’s circuitry, and the secondary
      side, isolated from the AC line by the transformer and connected to circuit ground in
      the rest of the device. Linear supplies share the same basic organization, except that
      there’s no circuitry on the primary side beyond a fuse.
           The primary side of a switcher is where most of the trouble is, and also most
      of the danger. The chopper circuit stresses its transistor harder than any other
      component in the supply, leading to frequent failures. The side’s direct connection to
      the house wiring makes service hazardous because any contact between you and
      ground completes the circuit. The secondary side is at lower voltages, and its isolation
      from the line means it’s a lot safer. Some switchers go as far as having holes in the
      circuit board between sides for some extra protection from arcing over and loss of
      isolation.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 235

  How to Fix One
  Most switchers fail from bad electrolytic capacitors, blown rectifiers or a dead chopper
  transistor. Look at the capacitors first. Any bulges? Change them. Leakage? Change
  them. Anything at all unusual about their appearance? Change them!
       Checking the rectifiers is easy enough if they’re separate diodes. When you have
  a bridge rectifier, with all four diodes in one package, each diode must be tested as if it
  were a separate part. Take a look at the bridge rectifier diagram back in Chapter 7.
  With all power disconnected and the big electrolytic near the bridge discharged,
  desolder the bridge from the board and use your DMM’s diode function to test each
  diode in it. You should see around 0.7 volts drop at each diode in the forward direction
  and an open circuit in the reverse direction, as with any silicon diode. If you find an
  open or a short in any of the diodes, replace the bridge.
       The chopper is the big transistor, probably heatsinked, on the primary side of the
  transformer. Some choppers are bipolar transistors, but most are power MOSFETs.
  If the fuse is blown, it’s a good bet the chopper has shorted out. The transistor can
  fail open, too, in which case the fuse might still be good. The transistor may have
  shorted and then opened, and the fuse may or may not have survived the momentary
  overcurrent. It’s an old technician’s anecdote that transistors are there to protect
  fuses! Check the chopper using the out-of-circuit techniques discussed in Chapter 7.
       If you have an isolation transformer, you can do some powered tests before
  pulling parts. Check the voltage across the big cap on the primary side of the supply,
  near the chopper. Remember that you can’t use circuit ground on this side. The
  negative terminal of the cap will be your reference point, where you’ll connect the
  meter’s black lead. You should see at least 300 volts. If it’s much less, suspect a bad
  bridge rectifier. If it’s zero, the fuse is probably blown, which could mean a bad bridge,
  a shorted cap or a bad chopper.
       It’s best not to try to scope the chopper directly, as the voltages are very high.
  The safer approach is to scope the secondary side of the transformer, using normal
  circuit ground. Many switchers have multiple taps on the secondary winding. Any of
  them will do, as long as it’s not the one connected to circuit ground. If the chopper
  is running, you’ll see pulses at a significantly lower voltage than what’s on the other
  side. They won’t be tiny, though. Expect anything from 10 to perhaps 60 volts from
  the baseline to the peak. No pulses? She ain’t running.
       If the chopper is good but isn’t running, suspect the pulse-width modulator
  (PWM) chip or the regulation circuitry near the output. Open zener diodes on the
  secondary side can allow the output voltage to rise too high, activating protection
  circuitry and shutting down the PWM, or even tripping the crowbar, deliberately
  blowing the fuse. No pulses, no chopper, no operation.
       If the supply is running but not putting out proper power, caps on the secondary
  side are the primary suspects. Scope them. If you see much of anything but DC on
  an electrolytic that has one lead going to ground, change it. Either its capacitance has
  declined, its ESR has risen, or both. If you change the cap but the waveform still looks
  noisy, look for a leaky diode feeding the cap.
236   How to Diagnose and Fix Everything Electronic

          Finally, remember that most switchers will shut down if output current demand
      exceeds their safe limits. Some may blow their fuses for the same reason. A short
      somewhere else in the machine may be pulling too much current and causing the
      supply to act like it’s broken.



Audio Amplifiers and Receivers
      Audio amps and receivers are at the centers of all home theater setups. The units
      have to produce significant power to drive speakers, so they include a fair amount of
      heat-generating circuitry prone to failure.


      How They Work
      Though today’s audio amplifiers and receivers employ digital signal processing
      for surround sound decoding, delay effects and sophisticated tone controls, power
      amplification is still an analog process in nearly all of them. The exception is the Class D
      digital amplifier, which converts the incoming signals to pulse-width modulation of a
      high-frequency carrier. The pulses are then current-amplified, with a lot of amperes
      available in each pulse. At the output, a smoothing filter blends them back into audio
      before sending them to the speaker. Class D amplifiers are used mostly in automotive
      applications because they are extremely efficient and can develop a lot of power in a
      small box without getting very hot. Fidelity can be quite high in Class D, but achieving
      it isn’t easy. So, home audio gear, which lives in a quiet environment more conducive
      to critical listening, has stuck with analog amplification, a very mature, refined
      technology capable of exceptionally good sound reproduction.
           Before home theater, the chain was simple: preamp, through tone controls, to
      drivers and outputs. Not anymore! Input may come from analog jacks, digital coaxial
      or digital optical cables, with varying sample rates and bit depths (number of bits per
      sample). Several formats of multichannel encoding are used, too, and the unit has to
      be able to handle them all. Once the desired signal processing has been accomplished,
      the data is converted back to analog and applied to conventional audio circuits.
      Capacitive coupling, with each stage connected to the previous and next stages via
      capacitors, is rarely used because the caps cause phase shift and rolling off at the
      low end of the audio frequency spectrum. Most of today’s amplifiers are directly or
      resistively coupled, for maximum fidelity.
           Because DVDs brought audio with more than two channels to the home, the
      conventional stereo receiver is all but gone; newer units have at least five channels:
      two front, two rear and one subwoofer for deep bass. Some have seven. What’s the
      difference between and woofer and a subwoofer? One of them can operate under
      water! No, seriously, a subwoofer is for reproduction of only the lowest frequencies,
      usually under 100 Hz, while a woofer’s range may extend into the hundreds of
      hertz. With today’s small “satellite” speakers unable to reproduce low frequencies
      much at all, the subwoofer is really a woofer, but the term has stuck. One defining
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 237

  characteristic is that there’s only one of them, as opposed to the usual separate woofer
  for each channel, because very low audio frequencies have little directionality, filling
  the room regardless of the speaker’s position. Thus, there’s no separation, stereo
  or otherwise, and the sense of spatiality comes from the higher frequencies being
  reproduced by the satellites.
       For every channel there is a complete amplifier chain culminating in an output
  stage. Many receivers use power amplifier modules for their outputs, but higher-end
  units still go with discrete stages because they’re reputed to sound better.


  What Can Go Wrong
  The power supply works mighty hard, at least when the volume is turned up high.
  Moving a lot of air with the subwoofer, necessary for the serious bass frequencies
  found in movie explosions and such, takes an especially large amount of power. While
  modest home systems may have a hundred watts available for that, self-amplified
  subwoofers with several thousand watts have been marketed. Of course, people
  utilizing the full power of those things can’t actually hear their movies anymore, but
  that’s what subtitles are for, right?
       Plenty of receivers still use linear power supplies because switchers can introduce
  high-frequency noise that’s hard to eliminate. To power five channels of 100 watts
  each takes some serious iron in the transformer, along with high-current diodes,
  hot-running linear voltage regulators and huge electrolytic capacitors. There’s a reason
  receivers weigh so much! All that heat and high current take their toll, especially
  on the diodes and capacitors. Even with the generally higher reliability of the linear
  approach, power supply failures in receivers are common.
       The most trouble-prone parts of a receiver are the output stages. Whether
  modules or separate transistors, they are where the current is. Plus, they operate in
  their linear regions, neither saturated nor cut off at any time (one hopes!), so they’re
  essentially resistors, dissipating power supply current as heat. Look for large heatsinks
  and you’ll find the output stages.
       With all those jacks, input switches and interconnections between boards, signals
  can be impeded by bad connections, causing them to crackle or drop out completely.
  Phono preamp sections are especially vulnerable to this, as they handle the very tiny
  signals generated by magnetic phono cartridges, and it doesn’t take much to stop
  those. Cartridges put out around 5 mv peak-to-peak, compared to the 1-volt standard
  for line-level audio. Still, even with the higher-level signals, ratty connections in the
  signal path cause many receiver problems.
       Speaker protection circuits sense when there is significant DC offset, or variance
  at the midline of the signal waveform from 0 volts. When offset occurs, there’s a fault
  somewhere in the amplifier, and the protection circuits disconnect the speakers to
  prevent excessive power supply current from burning out their voice coils. At least
  that’s how they’re supposed to work. Now and then the protection circuit malfunctions,
  going into protection mode when nothing is really wrong.
238   How to Diagnose and Fix Everything Electronic

      Is It Worth It?
      Not much in a receiver’s power supply or amplifier chain is especially expensive or
      hard to get, so most receivers of any real value are worth repairing. Output modules
      are available from online parts houses, as are power transistors. You aren’t going to
      find the DSP (digital signal processing) chips, but it’s highly unlikely you’ll need them
      anyway. In receivers, the power-handling areas are usually where the mischief lurks.


      The Dangers Within
      Most output stages are complementary push-pull types fed by positive and negative
      power supply lines. (A push-pull design with only one power supply polarity is called
      quasi-complementary.) The supply lines may have 30 to 80 volts or so on them, so they
      are capable of shocking you. And should you be unlucky enough to touch both at the
      same time, you could come in contact with 160 volts or more.
          Receivers with fluorescent display panels have small inverters supplying the panels
      with the few hundred volts required to light them up. The inverters are typically located
      on boards just behind the displays.
          Heatsinks can get mighty hot, so avoid touching them if the unit has been on for
      awhile, especially at high volume.


      How to Fix One
      Servicing a receiver is very much a process of elimination. The first thing to consider
      is whether the problem affects all the channels. If so, head for the power supply.
      One channel could have a short pulling everything down, but the supply is the first
      thing to check. If only one channel is affected, that’s where you’ll find the trouble;
      the supply has to be okay, since it’s powering the other channels properly.

      Power Supply Problems
      With a linear supply, testing is pretty painless, compared to the ordeals of working on
      a switcher. The transformer comes early in the chain, and what’s between it and the
      AC line is easy to evaluate without power applied. If the unit is dead, check the fuse.
      As with other products we’ve examined, a blown fuse almost always means something
      pulled too much current through it. Look for shorted rectifiers or a short somewhere
      farther down the line, on the other side of the transformer. If there’s a connector you
      can pull to isolate the supply from the rest of the unit, try doing that to see if the
      overcurrent situation persists.

      Output Stage Problems
      A shorted output stage will pull the supply’s output down toward ground, possibly
      blowing the fuse. Sometimes there’s enough resistance between the shorted transistor
      or module and the supply to limit the current to a value below the fuse’s rating, so it
Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 239

  survives. The telltale sign of a shorted output stage is a loud hum through a speaker
  connected to that stage. However, other channels’ outputs may also produce the
  hum because they’re fed from the same supply line that’s being pulled down by the
  bad one. They won’t hum as loudly, though. Scope the output. If you see DC with
  something resembling a 60-Hz sawtooth wave riding on it, you’ve got a short.
      Frequently, changing the output transistors or the module takes care of
  everything. Sometimes, and particularly with discrete designs (no module), the driver
  transistors may be shorted too. Also, always check the resistors in series with the
  emitters of the power transistors in discrete stages. The overcurrent can really cook
  those babies, altering their values or even cracking them. Most techs replace the
  emitter resistors as a matter of course when they change output transistors.
      In direct-coupled designs, shorts in one stage may blow surrounding stages. I’ve
  seen a shorted output transistor take out every transistor in its channel, right back to
  the input jacks! That used to happen frequently in early designs, but it’s still possible
  even now. Ironically, the costliest units are more likely to have extensive damage,
  thanks to their closely coupled stages with few or no components to isolate them.
  That kind of circuit sounds the best when it works, but it’s a mess when it breaks.
      At the inputs to the driver stage, look for diodes or zeners. They’re used to set the
  bias points, keeping the positive and negative halves of the output stage just slightly
  turned on even when there’s no signal or it’s very small. An open zener or a shorted
  diode can make the bias go wild, causing the outputs to conduct themselves to death.
  Changing the transistors without checking the bias just means you’ll need more
  transistors in a few minutes.
      The damage can proceed the other way, too, from input to output. Again, designs
  with close coupling are the most susceptible because their output stages’ bias is set
  right at the start. If the input transistor shorts, it can drive DC right to the outputs,
  upsetting their bias and sending them to semiconductor heaven. Or, considering the
  heat, perhaps it’s the other place.
      Although it seems like diagnosing an amplifier should be very straightforward, the
  interdependence of DC levels from stage to stage can turn a romp into a nightmare. If
  you find yourself going around in circles, disconnect the output stages from the power
  supply and scope their input lines. You should see normal audio with some DC offset.
  Most receivers use bipolar output transistors, so the bias is a matter of base current,
  not voltage. With no power to the transistors, the offset may be significant, but it
  shouldn’t be close to the power supply rails.

  Small-Signal Problems
  Small-signal issues are like those in any other device. You could have a bad cap, a
  bad transistor, and so on. To determine whether the problem is in the small-signal
  sections, test the various inputs. If any of them works properly, the power supply and
  output stages are doing their jobs. The various signals—digital, analog, radio—wind
  up as analog audio at the inputs to the power amplifier stages, starting with the line-level
  (1 volt peak-to-peak) stage. If no input works, go back to the beginning of the signal
  path, using the simplest one possible: an auxiliary analog jack. Feed it a signal from
240   How to Diagnose and Fix Everything Electronic

      an audio player, select it on the front panel as the active audio source, and scope
      your way through the low-level stages to see where it stops. Oh, and be sure to turn
      the “tape monitor” switches off! They interrupt the audio path so a tape deck can be
      inserted in line with it. With no deck connected, the audio comes to a dead end, never
      reaching the power amplifiers.
           With their multiple input paths, receivers have more connections through
      switches than most devices, offering plenty of opportunities for bad connections.
      Newer units often have electronic switching, rather than the old rotary or pushbutton
      switches used for decades. Noncontact switching is more reliable, but a blown analog
      switch chip will stop things dead. It’s pretty rare for just one of the chip’s inputs to fail,
      though; a bad chip usually kills them all.
           Hum isn’t always a power supply or output stage problem. If the sound of the
      hum is thin, with high-frequency content that’s a little bit buzzy, and it isn’t loud
      enough to wipe out the audio, there may be a bad ground connection somewhere in
      the low-level circuitry. As in many products, grounding from the board to the chassis
      can be mechanical, provided by the pressure between the board’s ground lands and
      the metal tabs into which the board is screwed. Over time, oxidation increases the
      resistance of those connections, and hum can result.
           If the level of the hum varies with the volume control, the source has to be early
      in the chain, before the control. Try unplugging the audio source from the input jacks.
      If the hum disappears, you have a bad cable feeding the jacks or a ground loop between
      the audio source and the receiver. If it’s still there, pull the board and clean the ground
      lands and the chassis mounting tabs until they’re nice and shiny. If that doesn’t solve
      it, check for bad solder joints that may be causing poor grounding.



Disc Players and Recorders
      Though inexpensive, CD and DVD players are not simple. They are actually little
      computers, combined with the mechanical and optical sections necessary to retrieve
      data from the disc.


      How They Work
      To find, follow and decode the disc’s microscopic optical tracks requires a focused
      laser beam, a three-axis servo system and a fair amount of computing power. An awful
      lot has to go right for the data to be read from the disc and transformed into your
      favorite movie or music.
           First, the disc must be accepted into the player, properly seated on the spindle
      and clamped down. The machine moves the optical head to the center of the disc,
      where playback will begin. Then the laser turns on and the lens moves up and
      down while the phototransistors in the head look for a reflected beam from the disc.
      If no reflection is detected, the player stops and displays “no disc.” Once it sees a
      reflection, the machine stops the lens when proper focus is found. This is determined
      by reception of maximum beam strength in the head’s center detector with minimum
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 241

  beam strength in the side sensors, indicating optimum spot size. Only if focus
  is achieved will the player spin the disc and try to read the track. If the lead-in
  track is found, the sled motor starts moving the head away from the disc’s center,
  and playback begins. As the disc spins, its normal wobbles and eccentricities far
  exceed the size of its tracks, so the tracking and focus servos keep the lens dancing
  around in three-dimensional step with their movements. The disc’s speed gradually
  decreases as the head proceeds toward the outer rim, because the linear distance for
  each rotation increases. The object of the technique, called constant linear velocity,
  is to keep the speed of the track constant as it goes past the head, so the bits can be
  crammed in as tightly as possible for maximum disc storage capacity. The spindle
  speed is controlled by the microprocessor’s monitoring the rate at which data is being
  read from the disc. Only when all these systems work in concert can a disc be stably
  tracked and played.


  What Can Go Wrong
  The disc may seat incorrectly, resulting in more wobble than the focus and tracking
  servos can handle. Many tray-loading players drive the tray with a belt that stretches
  over time, preventing the door from opening or closing fully. They use nylon gears
  that can crack and stick whenever another gear’s mating tooth hits the crack. The leaf
  switch telling the micro when the door is open or closed can bend or become oxidized,
  so the door motor keeps running even after the door hits its limits.
       Portable players with top-loading lids have their own quirks. To prevent you from
  looking directly into a running laser, they use a small interlock switch to sense when
  the door is closed. A bad switch makes the micro think the door is open and results in
  no operation. This is a very common failure in these machines.
       The sensor indicating when the head is at the starting position can malfunction.
  Most players use a leaf switch, though some use an optical sensor. The optical variety
  is pretty reliable, but leaf switches get oxidized or corroded and stop passing current,
  so the microprocessor never gets the message that the head is positioned. The result
  is a clacking sound made by the head as it slams over and over into the end of the
  sled’s track.
       Unlike LEDs, laser diodes have finite life spans, and they get dimmer as they age.
  If the laser is too dim, the reflected beams will be hard to detect and decode properly,
  and the machine will have trouble starting discs. It may also skip on discs that do
  play, although there are other causes of skipping. And even bright lasers can develop
  odd internal reflection modes resulting in optical impurity; the beam stays bright but
  can’t be properly focused.
       If the ribbon cable’s connections to the head are oxidized, the weak signals
  from the photodetectors will be erratic, confusing the machine badly and causing
  symptoms much like those of a failing laser.
       If the sled motor has a flat spot on its commutator (where the brushes transmit
  power to the rotating coils), or the slide or gears need lubrication, the head will
  stick as it scans across the disc, resulting in skipping or freezing. It’s important
  to remember that the sled motor does not move the head in the tiny increments
242   How to Diagnose and Fix Everything Electronic

      required to advance it along with the moving track; such fine mechanical motion
      would be impossible to achieve in an affordable product, if at all. Instead, the lens
      moves sideways, even as it bobs up and down to maintain focus, until it approaches
      the point where the beam would miss the sensors. The micro detects when the
      lens is near its limits and pulses the sled motor, advancing the head. The lens then
      moves back to the center and the process begins anew. Essentially, the lens and the
      sled play a game of inchworm as they follow the recorded track across the disc. So,
      intermittent, tiny jerks of the sled motor are completely normal. If you observe the
      sled assembly of a working player, you’ll see the wormgear shaft twitch every few
      seconds as it pushes the head outward ever so slightly.
          If the spindle motor, which spins the disc, is worn out or gummed up, the disc
      may not come up to proper speed, resulting in a slow data rate the machine can’t
      process into normal audio or video content.
          And, of course, all the servo and decoding circuitry has to work properly. Playing
      an optical disc is a feedback process; the data rate tells the spindle motor how fast to
      turn, and the reflections from the disc surface tell the tracking and focus servos how
      to keep a grip on the track. A loss of any of these systems can keep the others from
      doing their jobs.


      Is It Worth It?
      If the laser is dead, forget about repairing the machine. The alignment of the laser
      diode in the head is critical, so you can’t pull the laser and replace it; you need a
      whole new head. When disc players were costly, replacement heads were available
      from parts houses. These days, the players are so cheap that there’s no market for the
      heads, so their sources have dried up.
           By the same token, if mechanical parts like motors or gears are broken, your only
      hope is a parts machine; you’re not going to find replacements.
           Disc players incorporated into game consoles are the notable exception. Game
      units still cost enough that heads and even entire sled assemblies are available. Check
      the Internet for parts houses supplying these items.
           Just about anything else can be fixed. You’re not going to find a source for large-
      scale-integrated (LSI) chips and such, but those are unlikely to be causing the trouble
      anyway, and you couldn’t change them even if you tried.


      The Dangers Within
      Never look directly into the laser beam! The wavelength used for reading CDs is in
      the infrared, but the purity isn’t perfect, so there is some visible red as well. The red
      portion of the output is dim compared to the primary infrared energy, so looking at
      it gives a false sense of what your eye is receiving. It’s like staring at a solar eclipse:
      your poor retina is getting blasted but you don’t know it. DVDs are played with a
      visible red laser. It’s just as damaging, but at least you can see what’s coming at you.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 243

  In either case, you might burn a permanent hole in your visual field if you look
  straight down the bore.
        It’s routine to check for a working laser, but always look from off to the side.
  There’s enough reflection in the lens to let you see if the beam is there. When you see
  how bright a DVD player’s beam is, even from the side, you’ll get a good sense of just
  how damaging a full-on view from either a DVD or a CD player could be.
        Under no circumstances should you ever try to view the laser of a disc recorder when
  it’s in record mode, even from off to the side! The optical energy output is high enough to
  pop balloons; imagine what even a momentary reflection might do to your eyes.


  How to Fix One
  The most common problems are failure to accept and seat the disc properly, inability
  to play at all, and skipping while playing. If the door won’t open, look at the display.
  Is the digital control system working? Do normal numbers or messages appear on the
  display? If not, head for the power supply and check all the usual things like capacitors
  and output voltages. If the supply is good, scope the clock crystal at the micro. Players
  typically have multiple crystals, but you should find one right next to the biggest chip
  on the board. See if it’s running. It should have a sine or square wave on it of at least a
  few volts, at or very near the frequency marked on the crystal.

  Door Problems
  Assuming the power and control systems are working, check for leaf switch problems
  around the door. The exact layout varies from machine to machine, but the door
  motor, gears and belt are usually located under the platform (the entire mechanism),
  and the leaf switch will be buried in there someplace, with wires going to it.
       Reaching the door mechanism requires removing the platform from the frame.
  Look for large-ish screws at the corners, seen from above. You’ll probably have to
  remove the front panel as well, although sometimes you can pull the platform toward
  the back and lift it out without doing so. See Figure 14-1.
       Some players use two leaf switches, one for the fully out position and one for
  fully in, but most use one single-pole, double-throw (SPDT) leaf. The center blade gets
  pressed against one outer blade when the door is open and against the other when it’s
  closed. Look for three wires. Check that the blades aren’t bent, and that their contact
  points are clean. If the contacts are black, their lubrication may have dried out and
  become insulative. Gentle application of fine sandpaper will take off the black coating.
       If the door opens and closes but not all the way, either the belt is shot or there’s
  a broken nylon gear. Nylon gears crack very easily. Any sort of abuse can break them,
  and sometimes they just fail with time. If the door is very sluggish, the belt is the
  likely culprit. If it moves at normal speed but stops abruptly at a consistent spot, look
  for a broken gear. Check between the teeth; sometimes a grain of sand gets in there
  and jams the gears without breaking anything.
244   How to Diagnose and Fix Everything Electronic




                 Figure 14-1      Door mechanism on underside of platform




      Clamp and Spindle Problems
      A working door mechanism should take the disc in, plop it on the spindle and then
      lower the clamp onto it. Believe it or not, the only thing keeping the disc in contact
      with the spindle is friction. The clamp contains a magnet that’s attracted by the
      metallic spindle, holding the disc in position and facilitating the friction grip.
           Portable players rarely have clamps. Instead, three or four spring-loaded ball
      bearings or tiny tabs in the spindle grab and hold the upper edge of the disc after it is
      pressed down firmly. I’ve never seen one of those be a problem.
           In door-type players, improper disc clamping is common. The clamp’s upper
      portion hangs loosely in its holder and should float when the disc is spinning. If the
      holder is misaligned, the clamp will rub against it, making an obvious noise and
      dragging the disc speed down, perhaps preventing playback. Don’t be surprised if
      the clamp wobbles at the top a bit without rubbing. That’s normal. Rubbing, of course,
      is not.
           Dirt on the spindle can keep the disc from sitting flat, leading to focusing errors
      and skipping. If it gets greasy, the disc may slip. Make sure the spindle is clean.
           When the disc spins, it shouldn’t wobble much. You may see some shimmering
      of the surface, but the edges should sit pretty flat. If it wobbles, try another disc, to be
      sure the first one isn’t warped. Assuming a good disc, any significant eccentricity is
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 245

  caused by either a bent spindle shaft (unlikely in door-type players but more plausible
  in top-loading portables, because people have to press on it when inserting discs) or
  clamping problems.

  Playback Problems
  If the disc is properly seated but won’t spin, the startup sequence of head positioning,
  disc detection and focusing has failed. In a door-type player, it should begin as soon
  as the disc is seated. In a portable unit, there’s another element: the interlock switch.
  When the lid is closed, a little plastic finger on it protrudes through a hole in the
  player’s body, pressing on the switch. If the finger breaks off or the switch fails, the
  micro will never know the door has been closed, and nothing will happen. I’ve seen
  numerous bad interlock switches in these players.
        Is the head all the way in toward the center of the disc? If not, there’s a sled
  problem. Check that the rails are greased at least a little, and that the start position
  limit switch is okay. Sometimes the leaf switch gets bent just a tad and will still work,
  but not until the head is closer to the spindle than it should be. Focus will be achieved
  but the head won’t find the lead-in track, and it’ll just sit there until the machine gives
  up and stops.
        Before going further, check that the lens is clean and not scratched. Portable
  players with exposed heads are especially subject to dirty and damaged lenses. The
  lenses in door-type machines are well protected from scratches, but they can still
  accumulate dirt. If the machine has been in a smoker’s home or was installed in a
  kitchen, there may be a film on the lens, preventing proper focus. Even a protected
  lens could be scratched if the user tried one of those nasty cleaning CDs with brushes.
        Getting to the lens in a portable is easy. Pop open the lid and there it is. Take
  a cotton swab and wet it with water. Do not use alcohol! The lenses are plastic and
  will be destroyed by it. Blot most of the water from the swab with a tissue, so that
  it’s damp but will not drip water into the optical head. Wipe the lens gently and then
  wipe it again with a dry swab. Don’t put pressure on the lens while doing this.
        To clean the lens in a door-style player, you may have to disassemble the clamp
  assembly and remove it. Sometimes you can reach under it by bending a swab’s head
  at an angle.
        With the head at the starting position, the lens should move up and down as the
  player searches for proper focus. The lens is mounted on coils of very fine wire called
  voice coils, so named because the arrangement of a coil over a magnet is similar to
  what’s found in a speaker. Current passing through the coils generates a magnetic
  field that interacts with a permanent magnet in the head, permitting the control
  system to move the lens toward or away from the spindle and up and down. During
  playback, the lens assembly floats on this field, bobbing and weaving as necessary to
  follow the disc’s track. Look at the head from the edge of the disc and you’ll see the
  lens. If it’s not moving, there’s a problem with the focus circuits or the cable to the
  head. Even with a dead laser, the lens should move a few times until the micro figures
  out there’s no reflected beam.
        If the lens doesn’t move, the start-up sequence has not been initiated. There could
  be a digital control problem, or the door’s leaf switch might not be signaling the micro
246   How to Diagnose and Fix Everything Electronic

      that the door has been closed. The door motor may still be straining, but you might
      not be aware of it. In portables, check that interlock switch! If it doesn’t tell the micro
      the door is closed, nothing will start.

      Laser Problems
      Is there a beam? Its side reflection will be red (except in a Blu-Ray player, where it’ll
      be blue). In a CD player, the beam will look dim. In a DVD player, it’s very bright.
      If it’s there, the laser is probably okay. It might still have problems, but at least you
      know it’s not dead.
            If there is no beam, you’ve hit on the trouble! Alas, most disc player failures are
      due to a bad laser. If the lens is moving but there’s no light, the laser is probably shot,
      and you have just become the proud owner of a parts machine. To be sure, you can
      trace back from the diode and see if it’s getting voltage. Laser diodes are driven by a
      few volts DC, and there’s usually a tiny trimpot right on the head at the diode that’ll
      help you find the right connections. If the DC is there but the beam is not, it’s bye-bye
      laser and bye-bye player.
            You may see two trimpots in DVD players. Many use two lasers, an infrared for
      playing CDs and a visible red for playing DVDs. If the machine will play one but not
      the other, one of the lasers may have died. Be sure to test the unit with the type of
      disc it won’t play.
            Even if there is a beam, it might be too dim for proper operation. It’s hard to tell
      with a CD player, because most of the energy isn’t visible anyway, but you can get
      a good idea by comparing the brightness to that of a good player. In a DVD player, the
      beam is so bright that often you can see it right through the disc!
            If the beam shines and the lens moves, but nothing else happens, it may not be
      finding focus. Look on the board for a test point labeled “FOK” (yeah, I know), “FOC
      OK” or “FOCUS OK.” Scope it. It’ll change state (usually from low to high) when focus
      is achieved. If it doesn’t, then something is preventing proper focusing.
            If the disc starts spinning, focus lock is good. The rotational speed depends on where
      on the disc the head is positioned. At the start, the disc should spin a few hundred
      RPM. If not, there’s a problem with the spindle motor. Either it is mechanically
      gummed up, the motor is bad, or the driving circuitry isn’t doing its job.
            Hair, both pet and human, can wind itself around the spindle motor’s shaft, even in
      the protected environment of a door-style player. Remove the disc and turn the spindle
      by hand. It should turn easily and smoothly. If not, check for hair. Sometimes the
      motor goes bad or its lubrication dries out. If the spindle offers significant resistance,
      there’s a mechanical problem of that nature.
            The optical head provides two functions: tracking and data extraction. To get data,
      tracking has to be working. Either or both of these can be affected by oxidation or
      corrosion of the head’s ribbon cable connectors. After a dead laser, this is the second
      most common head-related problem. Laptop drives, for some reason, are especially
      prone to this issue. Pull the cable at the board end and look at its metal fingers.
      If they’re gold, wipe them with an alcohol-moistened swab. If they’re silver, they
      probably have solder on them. These are the kind that cause the most trouble. With a
      magnifier, look for black pitting. Gently scrape it with the tip of an X-Acto knife and
Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 247

  then use a swab to wipe away the tiny metal flakes. Clean with alcohol and reinsert.
  If there’s a connector at the head end of the cable, do the same thing there.
       Be aware that laser diodes are easily destroyed by static charges. Be sure you and
  your tools have touched ground just before you begin working on the cable. Reinsert
  the cable and check to see if proper operation has been restored. I’ve saved countless
  laptop drives with this procedure.
       The primary output from the head is a signal called the eye pattern. See Figure 14-2.
  This signal is the actual, raw data being read from the disc. All players have a test point
  at the first preamplifier stage, which you can find by following the head’s ribbon cable
  back to the board. The preamp will be a chip of low to medium density, and you’ll see
  test points very near its pins. Often they’re not labeled, but you can find the eye pattern
  by scoping the points; no other signal will look anything like it.
       The eye pattern should be around 1 volt peak-to-peak. If it’s much less than that,
  either the player is not tracking well or the laser is dim. Some amplitude wobble
  is normal as the disc spins, especially when the head is near the outer edge, but it
  shouldn’t be more than 15 percent or so. If the amplitude dips a lot, expect skipping
  or dropping out. Weak lasers can cause this, as can problems with the focus servo.
  In players that have focus gain trimpots, you can sometimes compensate for a less-
  than-optimum laser by upping the gain a tad. Most newer players don’t have servo
  adjustments, but it’s worth looking for a trimpot labeled “F GAIN” or “FOC GAIN,” just
  in case.
       If all looks well but the player skips, check for binding in the sled. If it’s well greased
  and clean, with no hair in the gears, there might be a tracking issue. With power off,




                    Figure 14-2     Laser optical head eye pattern of a
                    CD at a sweep rate of 100 ns. These are fast
                    signals! Use 50 ns for DVD eye patterns.
248   How to Diagnose and Fix Everything Electronic

      gently move the lens back and forth with a swab, checking that it moves freely. There
      could be dirt or hair there too.
          Some players have tracking servo adjustments. Look for “TR GAIN” or “TRACKING
      GAIN” and try increasing it a little bit. The more hissing noise the lens makes, the higher
      the gain. You’ll hear a “knee” above which the hiss will suddenly increase a great deal. Be
      sure to stay below that point or irregular tracking may occur.
          If the machine is tracking the disc and the spindle is turning at the proper rate,
      there should be normal playback. Any other problems will be due to circuit failures
      that are probably more trouble to find than the machine is worth. Those kinds of
      problems are rare, though. Most players can be fixed with the procedures we’ve just
      examined.



Flat-Panel Displays
      Unlike the old CRT technology, today’s flat displays are matrixed. Each pixel, or
      picture element (a single dot), is addressed in X-Y fashion, so all of the circuitry
      required for scanning an electron beam over a screen is gone, and the associated
      service issues are different as well.


      How They Work
      In an LCD, a low voltage twists the molecules of a tiny bit of liquid-crystal material
      in each pixel, changing the orientation of its light polarization. In conjunction with a
      fixed polarizer at the front of the panel, that change of polarization darkens a pixel,
      making the pixel block the backlight’s illumination. In a plasma display, a high voltage
      causes ionization of gas in each pixel, generating ultraviolet light that excites colored
      phosphors to generate visible light.
           Today’s displays have millions of pixels. A full HDTV display of 1920×1080
      resolution contains 2,073,600 pixels, each with 3 subpixels of red, green and blue.
      That’s 6,220,800 tiny dots, and every one has its own connection! How are all those
      addressed? There aren’t millions of wires coming out the back, after all. The process
      works somewhat as it does with memory chips: row and column addresses are sent
      through decoders that fan out to the appropriate connections, eventually reaching
      each and every pixel through transparent, printed conductors at the edge of the glass.
      Using a grid formation allows far fewer connections than there are pixels; those 2
      million pixels require just 6840 lines (1920 dots × 3 colors, plus the 1080 lines to select
      the rows). At the edges of the panel, the decoder chips make contact with the glass
      elements via ribbon cables affixed with pressure and conductive glue.
           Virtually all LCDs made today are of the TFT, or thin-film transistor, variety.
      Instead of addressing the LCD elements directly, the exciting voltage pulses a
      transparent transistor behind each element. That enhancement lets the pixel store
      its state after it’s been addressed, resulting in much higher contrast than if it only got
      pulsed and then left alone until the next frame of video came along.
           Printing millions of functional transistors over the area of an entire screen
      requires very high-precision manufacturing. In the early years, TFT LCDs suffered
Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 249

  from bad pixels; most had a few, and it was considered normal. Today’s displays
  rarely ever show any stuck pixels. Nearly all panels, LCD or plasma, are 100-percent
  functional. It’s pretty amazing, really.


  What Can Go Wrong
  Loss of a single connection out of thousands at the panel’s edge results in an entire
  row or column that won’t darken, leaving a bright line across or down the screen.
  Individual pixels can also fail, resulting in one dot of color that never moves on an LCD
  or a dark spot on a plasma. Plasma sets burn their phosphors when bright images don’t
  move for an extended period, reducing the brightness of the affected pixels and leaving
  a ghost of the offending image. That happens most often with panels used for static
  display of airport schedules and such, but it also occurs at home with extended video
  game play and TV network “bugs,” or logos, at the bottom of the screen.
      The fluorescent backlights in LCDs are driven by inverters producing a fairly high
  voltage. They’re just like the inverters in laptops, only bigger. Since the illumination
  provided always has to be as bright as the brightest picture could get, the inverters
  run pretty hard and hot, and are failure-prone. Some new sets use LED backlighting,
  eliminating the inverters. Those should last a good long time.
      The high-voltage–generating circuitry in a plasma set also works very hard. Plasmas
  run rather hot, too, so they’re more likely to experience thermal breakdown.
      The power supplies providing the low-voltage, high-current power to run all this
  also generate some significant heat, leading to shortened component life and failure.


  Is It Worth It?
  If the panel develops a bad row or column, forget it. There is no way to repair that.
  I fixed a few early LCDs by making a plastic clamp to squeeze the ribbon cable’s
  conductors against the glass, but in today’s higher-density panels they’re inaccessible,
  and the size scale would likely make such a crude repair impossible anyway.
       If the glass is damaged, you can’t repair it. Plenty of today’s TVs get hit by a kid’s
  toy or the family dog, rendering the sets useless. The cost of a new panel is usually
  more than the price of the TV.
       Electronic problems like bad power supplies, blown inverters and failing capacitors
  can be successfully navigated. Luckily, those account for most of what you’ll see.
  Computer monitors, especially, spend thousands of hours turned on, with the expected
  degradation of capacitors.


  The Dangers Within
  Plenty! In LCDs, watch out for the inverter and its output cables. They may have more
  than 1 KV on them. Plasmas are full of high voltage too, and it’s fed to the panel’s pixels,
  not just to a backlight lamp or two.
      The actual liquid crystal material in an LCD is toxic and should not be handled.
  You’d come in contact with it only if the glass were broken.
250   How to Diagnose and Fix Everything Electronic

      How to Fix One
      In plasma sets, look for bulging electrolytic power supply caps and bad connections.
      A total failure might indicate a blown chopper or other typical switching power supply
      issue. Beyond those, much of the set is made of specialized, high-voltage parts best
      left alone, unless the trouble is in a small-signal area like the tuner.
           LCDs also have the usual power supply and cap problems, but their most frequent
      cause of failure is the backlight inverter. If you’re not sure what an inverter looks like,
      see Figure 10-5 in Chapter 10. LCDs of any significant size have at least two lamps
      driven by multiple inverter circuits, often combined onto one board with output
      transformers at the ends.
           If you turn on the set and it lights up for a second before the screen goes dark,
      one of the inverters has died. You get that one moment of light because the other
      section is running its own lamp until the micro senses the loss of the blown one and
      shuts them both off a second later.
           The poor output transistors in an inverter work like dogs for countless hours, and
      eventually one of them shorts, taking the fuse on the inverter board with it. In most
      designs, each side will have its own fuse soldered to the board near the connector
      from the power supply or the microprocessor board. That’s very helpful, because
      a blown fuse tells you which side of the board has quit. Try replacing the fuse first,
      even if just with a temporary arrangement employing two soldered wires, clip leads
      and a physically larger fuse of the same rating as the original. If you can’t determine
      the original’s rating, 3 amps is a reasonable value to try. Now and then you may find
      that the fuse has fatigued and failed but the rest of the circuitry is fine. If the lights
      come on and stay on, a new fuse is all you need. Be sure both lamps are working.
      The screen should be at normal brightness and evenly lit. If one end is significantly
      brighter than the other, one lamp is still out, and it’s quite possible that the transistors
      on the blown side of the inverter became open from the momentary surge current
      when they shorted. So, they won’t blow a new fuse, but they won’t work either.
           Take a peek at the inverter’s transformer. If you see a burned spot anywhere,
      the transformer’s insulation is damaged and the coil has arced over, either from one
      winding to the next or from a winding to the core. The increased current draw from
      arcing usually pops an output transistor. Changing the transistor does you no good,
      since the transformer will just kill the new one. If you see no burns, the transformer
      could still have internal shorts or arcing, but it’s less likely.
           If the transformer looks okay, you can change the inverter’s transistors if you
      can find some. Often, they’re oddball output components that aren’t easy to locate.
      Sometimes you can substitute similar transistors, but they have to be a pretty close
      match, especially in their gain characteristics. Not enough gain will cause the transistors
      to run more in their linear region than fully saturated, and they’ll get very hot and fail
      in a hurry. Too much gain can cause them to “ring,” with the tops and bottoms of what
      should be a square wave having sine wave-like variations. That also puts them in their
      linear region and overheats them. If you do sub the transistors, scope their collectors
      or drains and compare what you see to the waveforms on the good side of the inverter.
      If they look a lot different, those transistors are not a suitable match. As long as the
      waveforms look like they’re turning on and off all the way, and the inverter seems to
  Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 251

    work, let it run for a few minutes and then turn it off and touch the transistors.
    They might be warm, but shouldn’t be too hot to touch. Compare them to the
    good side.



Hard Drives
    Hard drives are in everything these days, from computers to MP3 players. Most
    people consider the drives irreparable—they either work or they don’t—but that’s not
    always true.


    How They Work
    Hard drives use a rotating platter coated with ferric material onto which is recorded
    digital data in the form of tiny regions of magnetism. The head flies on a cushion of
    air just a few wavelengths of light from the surface, suspended on an arm that flits
    at high speed around the disc, controlled by the drive’s microprocessor as it reads
    and writes sectors of data. Unlike the heads in a video recorder, a hard drive head
    never touches the recording medium. The disc speed is held constant by a servo that
    monitors motor speed and locks it to a crystal reference.


    What Can Go Wrong
    The primary cause of hard drive failure is a head crash. The drives are well sealed,
    but even one very tiny piece of foreign material can stick to the disc surface, causing
    a disastrous scratch when the head hits it.
         Wear or lesser scratching of the disc surface can result in bad sectors, areas of
    the disc that won’t reliably return the data written to them. Most drives have a few,
    and they are hidden by a lookup table in the drive’s ROM that avoids the known bad
    ones. If enough bad sectors accumulate, the drive is failing and will have a hard time
    processing data.
         Stiction, the adhesive attraction caused by molecular forces between very highly
    polished surfaces, can make the head stick to the disc, especially in older, well-worn
    drives. This used to be much more of a problem than it is with modern drives. The
    symptom is that the drive has a hard time starting up. If it won’t spin on its own but
    will start after a good slap, the head is probably sticking.
         Another cause of hard starting is moisture inside. Because the flying of the head
    over the disc requires air pressure equalized to the outside world, hard drives have a
    small “breather” hole. The air coming in is carefully filtered, and a desiccant inside
    the drive absorbs moisture. You can hear its crystals moving around on some drives
    when you shake them. Eventually, the desiccant saturates and moisture builds up on
    the disc, crashing the head or making it stick.
         The motor or its drive circuits can fail, in which case the disc won’t turn no
    matter what.
252   How to Diagnose and Fix Everything Electronic

         The signals from the heads are very tiny, and a poor connection between the
      body of the drive and the heads can cause them to drop out or get too weak to read.
      The drive will recalibrate with a clacking noise, desperately trying to find the data. It
      may also write incorrectly, severely corrupting itself.


      Is It Worth It?
      Is there really anything you can do to repair a hard drive? Opening the case is out of
      the question; once you allow room air in, the drive is wrecked. Believe it or not, there
      is one thing you can try, and I’ve saved numerous drives with the procedure. It’s fast
      and easy, and it doesn’t involve breaking the seal.


      The Dangers Within
      No danger here. Voltages are low, and all the moving parts are sealed.


      How to Fix One
      If the drive is recalibrating often or returning errors, take off its circuit board and look
      at the connection points interfacing the board with the body of the unit. You’ll see
      two sets—one for the motor and one for the heads. Rarely are the motor connections
      problematic. The signals from the heads, though, are so small that it doesn’t take
      much resistance to lose them. See Figure 14-3.
           Some brands of drives use lovely, gold-clad connectors that almost never cause
      trouble. Many manufacturers, though, save a few cents by replacing the connectors




                                                                          Pitted contact points




                                                                          Contacts to heads




            Figure 14-3      Hard drive head contacts
  Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 253

    with sharp pins on the body of the drive that press into solder pads on the board.
    After a few years, the solder gets oxidized and its resistance goes up, impeding the
    delicate signals generated by the heads when they read data. If yours has the pads,
    look at the indentations made by the pins. Are they blackened at the center? That’s
    the sign of this malady. Gently scrape off the oxidation with the tip of an X-Acto knife,
    and then wipe the pads with a dry swab to remove the metal flakes. Reassemble the
    drive and be prepared to be surprised! The darned thing just might work. If the drive
    has corrupted itself too much, it may be unrecoverable even though the electronics
    are now functional. Sometimes you can restore it only by reformatting, wiping out
    the data. Other times nothing works. But, heck, it wasn’t doing you any good before
    anyway, right?



Laptop Computers
    Laptop computers have largely replaced desktop machines in many people’s homes
    and workplaces. They offer numerous advantages in terms of required space, power
    consumption and heat generation, but they’re a lot more fragile.


    How They Work
    Laptops are functionally just like desktop computers, except that everything is much
    smaller and runs on less power, and the LCD monitor is built into the unit. Also, laptops
    incorporate power management systems for efficient charging and use of batteries, and
    their AC power supplies are not internal, except on some really old models.


    What Can Go Wrong
    Laptops are enormously complex, with most of the circuitry on the motherboard.
    That board has many layers and lots of LSI chips, along with a zillion tiny support
    components crammed together.
         Some of those huge chips are connected to the board with a ball grid array, or
    BGA, which is a bunch of tiny, ball-shaped contacts soldered to pads underneath the
    chips. If you see a chip with no leads, it has a BGA. See Figure 14-4. BGAs provide
    hundreds of contacts in a small space, so they’re used for microprocessors, video
    graphics chips and other very high-density devices. Some of those items run pretty
    hot, unfortunately, and can degrade their solder joints with flexure of the board and
    time, resulting in intermittent connections and a machine that keeps crashing.
         How can you repair those things when you can’t even see the contacts? It takes
    specialized rework equipment costing in the range of $50,000 to remove and resolder
    a BGA. There’s just no way to do it at home. I foolishly tried resoldering one barely
    visible pin at the edge of a video graphics chip’s BGA once, and succeeded only in
    destroying the chip and the motherboard. Fuggedaboudit.
254   How to Diagnose and Fix Everything Electronic




                 Figure 14-4     LSI chip with BGA contacts underneath


            Many motherboards sport a surprising number of surface-mount electrolytics,
      with the usual problems those cause. When changing them, be extra careful with the
      heat, since you’re dealing with a multilayer board you could easily wreck. Review
      Chapter 12 for info on handling situations like that.
            Battery charging is controlled with MOSFET power transistors. An open one will
      result in no charge reaching the battery. All modern laptops use smart batteries with
      their own microprocessors that tell the machine the state of charge, how many cycles
      have been used over the life of the pack, the battery’s model number, and so on. If
      that micro gets scrambled, or the contacts between it and the laptop malfunction, the
      battery may not charge or even be recognized. Of course, worn-out cells will cause the
      same problems.
            The power supply input jack is a frequent source of laptop misery. Pulling on the
      plug, tripping over the cord, and even just normal insertion and removal can crack
      the jack’s solder joints, resulting in failure to charge the battery or no AC operation at
      all. If you have to wiggle the plug or push it to one side to make it work, the joints are
      cracked.
            A dead backlight inverter is one of the most common laptop problems of all. The
      latest-generation machines use LED backlights, so they have no inverters, but most
      laptops out there still have fluorescent lamps. The lamp itself can get weak, and dropping
      the machine may break the bulb with no outward physical sign of damage. If the laptop
      has been dropped and suddenly won’t light up, but you can see the image by shining
      a bright light on the screen, the long, thin lamp tube is probably shattered inside the
      bottom of the LCD.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 255

       The screen is connected to the body of the machine via cables running through the
  hinges. After the lid has been opened and closed hundreds of times over a few years, a
  cable can break. In most models, the inverter is located in the screen assembly to avoid
  having its high-voltage output wires going through the hinges. The power and control
  lines feeding it do go through them, and those are often the ones that break.
       A bad RAM (memory) module can cause random crashes. Sometimes the module
  isn’t actually defective, but its contacts have gotten oxidized, resulting in weak signal
  transfer to the motherboard. Bad RAM soldered on the motherboard will cause
  crashes too.
       Usually, the hard drive and the optical drive coexist on the same system bus. If one
  of them malfunctions and hogs the bus when it’s not being addressed, the machine
  will hang.
       The keyboard can develop bad keys or entire bad rows. It can also drive the machine
  crazy with a stuck key that sends the same signal indefinitely.


  Is It Worth It?
  A bad motherboard is usually not worth your time, but there are exceptions. If you
  see signs of dying electrolytics, you can change those and probably restore normal
  operation. If the battery won’t charge but everything else works, and you’re certain
  the battery is good, you may be able to find the bad MOSFET and replace it. Those are
  big enough to remove and resolder. If there’s some obscure logic failure or a bad BGA
  connection, it’s toast.
       Power supply jack problems are easy fixes, once you get to the darned thing! You’ll
  spend far more time on disassembly than on the repair. In some laptops, removing the
  back gets you right where you need to be, but others require total disassembly to get
  anywhere near those precious solder joints.
       Repair of a backlight inverter is tough, mostly because the tiny output transistors
  they use are hard to find, as are the soldered-in fuses. Replacing the inverter, however,
  is pretty easy on most models. Usually, all you need to do is open up the screen
  assembly, and there she is! Unplug the connectors, replace the board, and you’re
  done—that is, unless the problem is a broken screen cable.
       Ah, the screen cables. Very often the real reason the LCD won’t light up is because
  the inverter’s cable is broken. The problem can also be caused by a failure at the video
  graphics chip’s BGA. So, it can be tough to tell whether the inverter, the cable or the
  motherboard is the true culprit. If moving the screen through its range of angles makes
  it turn on and off, one of the cable’s wires is broken inside, and the ends are touching
  each other just enough to make contact when the cable is at certain positions.
       Bad RAM can be easy or impossible, depending on whether it’s a module or
  soldered to the motherboard. If it’s soldered on, repair is unlikely.
       Hard drives and optical drives are easily changed in some machines and hard to
  reach in others. The drives fail often enough that many manufacturers make them
  readily accessible, but some disregard that reality and bury the darned things so deep
  that it takes an hour or two to get to them.
256   How to Diagnose and Fix Everything Electronic

      The Dangers Within
      Most of a laptop operates at low voltages, so it’s pretty safe. The backlight inverter
      puts out a high voltage, so stay away from its output area. Switching converters on
      the motherboard can generate some other voltages that it’s best to avoid, but they
      shouldn’t be high enough to injure you. You could possibly get a shock if you touched
      the wrong point while the machine was running.


      How to Fix One
      Let’s look at a few common laptop problems and how to approach their repair.

      Crashes
      If the machine works but crashes randomly, pull the RAM modules and use a dry
      swab to clean their contacts on both sides. Even if they’re gold-clad, which most are,
      the swab may show a surprising amount of grayish dirt when you’re done. Pop the
      modules back in and test.
           If that doesn’t solve the problem, use diagnostic software to check for a bad RAM
      module. If one comes up as bad, replace it. That’s about all you can do. RAM is pretty
      reliable, but it does fail now and then. Some motherboards have RAM soldered on,
      with modules used only for expanding the memory above the stock configuration. If
      the motherboard RAM is bad, you’re pretty much stuck. If it’s an especially expensive
      laptop, you might be inclined to try replacing a RAM chip, but the size scale makes
      soldering difficult. Some of those chips aren’t so tiny that it’s impossible, though.
           If the machine crashes so often that you can’t even run the software, check for
      bulging or leaking capacitors on the motherboard.
           Some laptops crash after they warm up. Usually, that means a bad chip or an
      intermittent solder joint, probably in a BGA somewhere under the microprocessor or
      the video graphics chip. I’ve seen hard drive controller chips do that too; the machine
      works fine until that chip warms up enough to malfunction, and then it won’t read the
      drive or it corrupts the data.

      Charging Problems
      If the battery won’t charge, check first to see if the battery is any good. The cells’
      chemistry wears out eventually. Also, the internal charge control circuitry can fail.
      Sometimes a battery stored so long that all of its charge has leaked away may refuse
      to start up because its internal micro won’t signal its existence to the laptop without at
      least a little power to run. Some systems offer a battery resetting utility that can hunt
      for a dead battery and try some charge to see if it’s there and working.
           Short of cracking the battery open and applying a little charge directly to the
      cells, there’s nothing you can do. And, frankly, I don’t recommend doing that. It’s
      rather difficult to accomplish without wrecking the battery, and the lithium-ion cells
      inside are pretty dangerous if a screwdriver pierces them while you’re breaking open
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 257

  the plastic casing. Any breach of their seals can cause a fire hazard because lithium
  reacts violently with water, including water vapor in the air. Unfortunately, applying
  power to the pack’s outside terminals won’t work; the charge controller inside allows
  connection to the cells only when the laptop gives it the go-ahead.
        If you’re sure the battery is good, check to be sure the power supply is working!
  Many times the adapter fails, and it’s just assumed that the computer is the culprit,
  when in fact it’s not getting any power. If the adapter has an LED, it should be lit.
  Those LEDs are pretty much always driven by the adapter’s output. So, if the light
  comes on at normal brightness, the supply is working. If it doesn’t light up or is very
  dim, unplug the adapter from the computer and see if it comes back to normal. If so,
  there’s a short in the computer dragging down the voltage. If not, either the supply is
  dead or its output cable has a short. I’ve seen that happen a few times, and repairing
  the cable restored normal operation, thanks to protection circuitry in the supply that
  prevented its destruction or a blown fuse from overcurrent.
        Once you’re sure the supply works, check for a jack issue on the laptop and in
  the supply’s plug. Does the jack move when you wiggle the plug? Does indication of
  charging come and go when you do that? If you’re watching for software indication
  of charging, remember that it may take 10 or 20 seconds for the operating system
  to recognize the change. Lights on the battery itself, if there are any, are a far better
  indication of whether it’s receiving voltage. Even those may take a few seconds to
  respond, though, so wiggle slowly.
        To ascertain whether the trouble is in the plug or the jack, hold the plug steady
  with one hand while moving the wire with the other. If the connection cuts in and
  out, the plug’s the problem. If not, it’s the jack. You may have to do this a few times
  to be sure, because it’s easy to move the plug slightly in the jack while trying to hold
  it still.
        When you’re sure the power supply, jack and battery are okay, it’s time to
  consider a motherboard problem. Power management on laptops is complicated,
  involving firmware (software encoded onto the machine’s chips), system software and
  the power management unit, a specialized microprocessor used only for controlling
  the flow of power to the various parts of the machine. A problem with any one of
  these could prevent charging. If there’s a reset procedure for the power management
  unit, try that.
        If nothing works, open the machine and look in the area of the battery connector.
  Because significant current of up to several amps gets passed in charging, the power
  transistors controlling it are usually located near the connector to avoid having to
  waste space with wide circuit board traces that can handle the juice. The transistors
  could be on either side of the board. They’ll be bigger than most of the components
  around them. Use a shield or other obvious ground point, like a chassis screw
  obviously connected to a wide trace on the board, for circuit ground. With the AC
  adapter connected, try scoping the transistors’ terminals while you insert and remove
  the battery. If you find one with constant voltage on one terminal and a signal that
  changes state on the other when you pop the battery in, but nothing shows on the
  third terminal, the transistor is probably open.
258   How to Diagnose and Fix Everything Electronic

      Display Problems
      Unlike a desktop computer monitor’s DVI (Digital Video Interactive) or VGA (Video
      Graphics Array) connection, the interface from a laptop’s LCD to the motherboard is
      specific to the particular make and model. Signals may be carried on a bundle of wires
      or a printed-circuit ribbon cable. Ribbons rarely break from normal flexure, although
      some get brittle after a number of years. Who keeps a laptop that long anyway? Wire
      bundles do break, resulting in all kinds of display symptoms.
           Far and away, loss of backlight is the most common display failure. If you can
      boot up the machine normally and see the image on the screen by shining a bright
      light on it, the backlight has died. The three main causes are motherboard failure,
      cable breakage and a blown inverter. The fluorescent lamp may have broken if the
      machine took a fall, but that’s less common.
           This assumes there is a lamp. Even in newer laptops using LED backlighting,
      though, operation still depends on functioning cables from motherboard to screen.
           With the laptop running, gently move the screen back and forth through its entire
      range of angles. If you see even a flicker of backlight, the problem is almost certainly
      in the inverter’s cable. Most inverters have at least five wires going to them, and
      sometimes more. Usually, there’s +5 V, +12 V, on/off control, brightness and ground.
      Brightness is set by varying the duty cycle of a fast square wave. The higher the
      brightness, the longer the waveform stays up during each cycle. That arrangement
      allows the motherboard to control the brightness in purely digital fashion, with no
      varying analog voltages. If the wire breaks, though, the inverter will go dark, even
      though all other inputs are working. In fact, some models omit the on/off control wire
      and just turn off the pulse train to shut down the inverter when you close the screen
      or set the brightness to minimum. Loss of any of the other wires will kill the backlight
      too, of course.
           Inverters work hard; most the warmth you feel along the bottom edge of a laptop’s
      screen comes from the inverter’s output stage. So, as you might expect, the transistors
      blow. That’s especially likely when the screen has been run at full brightness for a few
      years. The heating and cooling cycle eventually kills those transistors.
           Unlike desktop displays, laptops usually have only one lamp and one inverter.
      Open up the screen assembly and you should see it. It’ll look just like the ones in
      desktop LCDs and TVs, only smaller. It may be covered in tape or with a shield. Very
      close to the inverter’s input connector (not the two wires going into the screen—that’s
      the high-voltage output for the lamp), you’ll find a fuse soldered to the board. With
      power disconnected, check it with your DMM. If the fuse is open, either the inverter
      has blown or the lamp is broken inside the LCD. Unless the machine got dropped,
      assume the inverter’s output transistors shorted and blew the fuse. As described
      earlier, you can try replacing those, but they’re hard to get. The easiest route is
      to replace the entire inverter board. Some manufacturers won’t even sell you an
      inverter; they want you to replace the entire screen assembly, which probably costs
      more than the laptop is worth. Check eBay and online parts houses for good deals.
      The first time I ran into this problem, I opted against the manufacturer’s $700 screen
      assembly replacement and found a new inverter online for $40. It worked just fine.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 259

       If the fuse is still good, a problem with the inverter is less likely. One of those
  pesky transistors could have opened and not blown the fuse, but they usually short.
  The cable may be broken, even if the swivel test didn’t turn anything up, or there
  could be a serious motherboard problem like a bad graphics chip or a broken BGA
  connection. Try pressing on the graphics chip. If the light flickers on, the machine
  crashes, or any other changes occur, there’s your answer: its BGA is intermittent.
       Testing the cable isn’t hard. Unplug it at both ends and use your DMM to check
  for continuity. To get a connection to those tiny holes in the connectors, connect a
  clip lead to a small component from your stash and push its lead into the connector,
  with the other end going to the DMM. Needless to say, you want to insert the same
  end of the component to which your lead is clipped! Just let the other end of the
  part hang.
       If the cable is good and the inverter’s fuse is good, it’s likely the motherboard
  isn’t turning the screen on, and you’re probably not going to be able to fix it without
  replacing the board. A lot of laptops get dropped or otherwise abused, and people sell
  them off for parts when the screen is cracked. The motherboard rarely gets damaged
  in a fall, so you may be able to scare up a parts unit online and swap out the boards.
  Just avoid buying any machine that’s had liquid spilled into it.
       If the screen lights up but the video isn’t normal, plug an external monitor into
  the machine and see if video works properly on that. If not, the motherboard has a
  serious problem at the graphics chip. If it looks okay externally, either the video cable
  or the screen itself is causing the trouble.
       A single bad line on the display cannot be the fault of the cable, because no one
  wire is specific to such a small area of the screen. A bad line or two is caused by the
  row and column drivers inside the LCD or their connections to the glass. You can’t
  fix this, but finding a reasonably priced replacement screen online is easy enough for
  many laptop models. Changing it is entirely a mechanical job; no soldering will be
  required. Take apart the screen bezel, get the LCD in the frame, plug in its connector
  and you’re done. You shouldn’t even have to open the main body of the computer.
       When video is severely distorted, with large areas of the screen a total mess,
  suspect the cable or the graphics chip. A bad screen can cause this too, but it’s
  less likely unless you see obvious cracks from a fall. I once worked on a laptop
  with video that started shaking back and forth after it had been on for about half
  an hour. Eventually, the image would tear, looking a lot like an analog TV with its
  horizontal hold misadjusted. I proved the fault was with the graphics chip by spraying
  component cooler on its heatsink. As soon as the chip cooled even a little bit, video
  returned to normal for a few minutes, until it got hot again.
       Before you give up, check the video cable the same way as with the inverter
  cable. A broken connection there will cause absolute havoc on the screen. Also, some
  LCDs have thin circuit boards on the back, with ribbon cables connecting them to the
  row and column driver chips along the screen’s edges. If they use sockets, check the
  fingers on those cables to be sure they’re not oxidized. Clean them and test the screen
  again. Beware the thin, printed conductors wrapping around the edges of the screen!
  Those are the connections to the drivers, and they’re especially fragile because of
  their density. Pressing on one is likely to ruin the LCD. Keep that in mind when
  installing a new panel, too.
260   How to Diagnose and Fix Everything Electronic

      Drive Problems
      As described in this chapter’s section on hard drives, there is an issue with some
      brands of drives, especially after they’ve been in use for a few years. The contacts
      from the board to the head assembly inside the metal casing get oxidized, causing
      read failures and sometimes severe data corruption. For some reason, laptop drives
      seem especially susceptible to this problem. If yours is recalibrating a lot, having
      trouble reading and returning errors, it’s worth taking it out and trying the procedure
      described in that section.
           The 5 VDC power to the drive needs to be quite steady and clean for the drive to
      function properly. Drives pull up to 500 milliamps, which is not insignificant. If the
      motherboard’s electrolytics are starting to get weak, the voltage may dip or develop
      spikes when the drive turns on, making it corrupt itself or causing malfunctions in
      other areas of the board. As the drive spins up from a dead stop, current draw can be
      a full amp for a second or so. See if malfunctions occur at the moment spinup begins.
      If so, suspect voltage regulation or bad caps on the motherboard.
           The optical drive eats around the same amount of current and can cause similar
      trouble. The information in the section on CD and DVD players, also in this chapter,
      applies to computer optical drives as well. In particular, the problem with oxidized
      ribbon connections to the laser head occurs more often in these drives than in shelf-
      style DVD players. If the drive has trouble reading discs, try cleaning the lens and the
      cable contacts. Very often, that’ll bring it back to health.

      Other Problems
      The Wi-Fi antenna cable on laptops with internal wireless cards goes through the
      hinges, with the antenna in the screen assembly. A broken coaxial cable will cause
      severely reduced wireless range. To check the cable, unplug it from the Wi-Fi card and
      use your DMM. Many machines have two antennas, with a small duplexing board
      in the screen assembly, so you can’t test from the card end of the cable right to one of
      the antennas. Be sure to check from the card connector to the duplexer. If you follow
      the cables from the antennas, which are little flat things next to the screen, they’ll
      meet at the duplexer. The third cable, going down to the hinges, is the one you want.
      Test from its solder contact on the duplexer back to the other end inside the main body
      of the laptop. Be sure to check both the shield and the center conductor, as either or
      both could be broken. Usually, it’s the center conductor that breaks.
           Laptop keyboards are mostly mechanical, with a conductive rubber button under
      each key. The connections are arranged in a grid, but the irregular layout can lead to
      some pretty obscure patterns. You can’t assume that it’s all rows and columns.
           When one key stops working, it’s a contact problem underneath. Time and
      oxidation can do it, but more frequently someone has spilled liquid into the keyboard.
      If you’re really intrepid, you might be able to take apart the keyboard and clean
      it out, but it’s a lot of trouble, and keyboards for most machines can be had pretty
      inexpensively. Replacing one is just a matter of popping it off, disconnecting the
      ribbon cable, and installing the new one.
  Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 261

         If a whole bunch of keys dies, and there hasn’t been a liquid spill, check the
    ribbon cable connections at the motherboard, because one entire line may be out. If
    not, there could be a motherboard problem with the keyboard decoder. Or, the break
    might be in the keyboard.
         To check it, remove the keyboard and do a continuity test on every combination
    of lines coming from the keyboard, keeping in mind that good contacts may show as
    many as a few dozen ohms, as is normal for conductive rubber switches. There aren’t
    that many lines, perhaps six or eight, so the test isn’t that rough. If a bad key produces
    no continuity on any combination, the keyboard is the problem.
         If the machine goes nuts, acting like someone is typing the same key over and
    over, that just might be the case! That “someone” was the person who spilled coffee
    or soda into the keyboard, shorting one or more of the contacts with conductive goo.
    Disconnect the keyboard and fire up the machine. If the stuck key goes away, you
    know somebody got sloppy with the drinks. This happens quite often, and the poor
    computer will act like it’s mondo loco, when all it really needs is some peace and
    quiet from its keyboard connector.



MP3 Players
    Being portable, MP3 players are subject to mechanical damage from being bounced
    around and dropped. The parts are small, making service challenging, but most
    players can be fixed.


    How They Work
    MP3 players retrieve blocks of data from a storage medium, either flash memory
    (the nonvolatile memory like that found in pen drives) or a mechanical hard drive.
    The data is quickly read into memory and then read back out at the rate required
    for conversion to uncompressed audio. When the memory buffer is nearly empty,
    another block of data is read from storage to refill it, providing continuous playback.
    The reconstituted audio data is fed to the digital-to-analog converter, or DAC, filtered
    to remove digital artifacts and boosted to headphone level with a small stereo
    amplifier.
         MP4 video players work the same way, except that they have decompression
    chips for video data, along with an LCD screen capable of video display.
         Overseeing all this is a microprocessor that selects tracks, minds the buffer, and
    extracts and displays information such as song title, artist and data rate.


    What Can Go Wrong
    Most MP3 player problems involve broken solder joints on their headphone and
    power jacks, failing batteries, cracked displays, bad hard drives and drive controller
    chips, and loose cables. People sometimes sit on the players, breaking the screen or
    crushing the case.
262   How to Diagnose and Fix Everything Electronic

      Is It Worth It?
      Repairing broken joints on jacks costs nothing but your time to disassemble the unit
      and resolder them. Batteries for popular, more expensive players are available at low
      cost online. Broken screens can be found there too, but if the case is badly bent or the
      circuit board is damaged, it’s not worth trying to repair the machine. The small hard
      drives used in MP3 players are a little more expensive than equivalent-capacity laptop
      or desktop drives, but they’re not prohibitive. A bad drive controller chip is irreparable
      without replacing the board, which is probably not worth the cost.


      The Dangers Within
      No dangers, except possibly from shorting the battery wires. Lithium-ion batteries can
      supply a fair amount of current into a short. The batteries get hot when they do, too,
      and could burst if the short isn’t resolved quickly.


      How to Fix One
      Opening the cases of some players is the hardest part. While some come apart the
      traditional way, with a few tiny screws, others are snapped together snugly and
      require a shim to separate the halves. Disassembly instructions for the more popular
      players are available online, as are plastic shim tools. Very often, the purveyors of
      batteries and drives for MP3 players include the tool with their products. Be sure to
      open the case gently, as ribbon cables may connect the halves.
           Before doing any soldering, disconnect the battery. Most batteries plug in on a
      connector, so pull it. Some players have directly soldered battery wires. Carefully
      unsolder the negative (black) wire and put a piece of electrical tape over the bare end
      so it can’t short against anything.
           If the hard drive is clacking and not retrieving data, the drive itself or the
      controller chip may be at fault. The only way to tell is to swap out the drive. The
      controllers use the same BGA connections that so many laptop chips have, with the
      same problem of broken solder joints under the components. There’s no way to fix
      them, but sometimes you can insert a shim of some sort to press against the chip and
      keep the connection working for awhile. That sort of “repair” doesn’t last long, though.
           Screen and drive replacement are straightforward. Be sure ribbon connector
      latches are securely closed; they tend to loosen in items that get bounced around. I
      like to put a little nail polish on the edges, at the latch tabs, just to be sure they stay
      put. If you do that, brush on only a small amount. You don’t want it dripping into the
      connector. Let it dry before closing the case, so the outgassing won’t remain inside.



VCRs and Camcorders
      Tape-based video recorders are complex machines with many interactions between
      their various sections. Servicing them can be fascinating and challenging.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 263

  How They Work
  VCRs and tape-based camcorders, both analog and digital, record very high-speed
  signals using the helical scan method, in which the tape is wrapped around a rotating
  drum, also called a cylinder. Protruding slightly from the drum are two video heads
  that scan the tape in a diagonal pattern, laying down very thin tracks next to each
  other, efficiently utilizing the width of the tape as well as its length to pack in lots
  of information. Helical-scan recorders are some of the most mechanically complex
  consumer devices ever sold. As a dying technology, they’re cheap now, but their
  high cost in the early years of home video recording was not from price gouging. The
  mechanical precision required in their manufacture, especially of the head drum
  and heads, was in the microns. Those babies were hard to make! It took 20 years of
  research and development to bring VCRs to the masses, and their eventual low cost
  was due only to the economy of scale.
       Analog recorders are more electronically complicated than their digital counterparts.
  First, the amplitude variations and noise inherent in weak, fast signals coming from tape
  necessitate the use of FM recording. The video signal is used to frequency modulate an
  oscillator, and the resulting RF signal is then recorded. On playback, FM demodulation
  extracts information only from the frequency variation of the carrier signal, ignoring
  noise and wobbling amplitude. As with FM radio, the technique permits an inherently
  noisy channel to provide a clean signal.
       TV signals are extremely time-sensitive, and no mechanical system can recover
  them from a recorded tape without timing errors rendering them from wobbly to
  useless. So, correction schemes involving phase-locked loops (PLLs) and, much
  later, digital timebase stabilization were developed. Color was especially difficult to
  reproduce, as even nanosecond-level jitters would wipe it out. A double PLL system—
  one for gross errors and one for fine adjustment—finally solved that problem.
       Tape isn’t perfect; it has microscopic areas where the magnetic oxide flakes off or
  is damaged by dust. On playback, the signal randomly drops out for very short periods
  as the heads lose contact with the tape. In an audio recorder, such dropouts are too
  small to be heard, and they also don’t extend across the width of the entire track,
  so they have no effect beyond slightly increasing the noise level. With the dense,
  narrow tracks of helical recording, dropouts trash entire lines of video. The dropout
  compensator, an analog delay circuit storing a line of video, fills in missing lines with
  the previous one, rendering most dropouts invisible.
       Digital helical machines only need to get bits on and off the tape, and they
  tolerate and correct timing errors by reading the data into memory and then clocking
  it back out steadily before converting it to an analog output signal. So, the timing
  correction systems in analog machines aren’t needed, and a simpler PLL system
  suffices. The data rate is quite high, however, and dropouts are still a problem. Digital
  error correction codes are used to calculate missing bits and fill in for what gets lost.
  The MiniDV format uses image compression similar to that in a JPEG, converting
  each frame to its compressed form and recording it separately. Newer formats using
  MPEG-2 or MPEG-4 compression periodically record key frames, which are complete
  frames, and then store only the changes between frames until it’s time for another
264   How to Diagnose and Fix Everything Electronic

      key frame. This more advanced technique results in far fewer required bits for a
      given picture quality level, but it’s less suitable to tape recording because lost bits can
      affect many frames, not just the one in which they occur. MPEG recording is used in
      virtually all card-style and hard drive recorders, though, because recovery of all the
      bits is pretty well assured, and its higher storage efficiency is crucial. A few MPEG
      tape camcorders have been marketed, but they’ve never caught on.
           In both analog and digital recorders, servos are used to control tape and head
      drum motion. In analog recording, the heads are time-aligned with the video frames
      by a head drum servo system to avoid having the machine switch from one head to the
      other during the frame. If you’ve ever rolled the vertical hold on a TV while playing
      a tape, you’ve seen a line of distortion just above the vertical sync bar. That’s the
      switching line, where the machine switched between heads while recording. Imagine
      that in the middle of the picture! Without a servo to keep it near the sync and off the
      screen, the ugly line would wander through the frame.
           To keep the heads centered on the tracks during playback, either head drum
      rotation or the tape motion through the machine must be controlled. Most VHS
      machines lock the heads to a crystal to keep their rotation steady, and control the
      pulling of the tape with a capstan servo, positioning the tape tracks under the heads
      as they fly by. It can also be done the other way, keeping the tape speed steady and
      adjusting head rotation, and some recorders use that method.
           In VHS, the video track position on the tape is sensed using a control track of pulses
      recorded along the edge of the tape, one pulse for each head drum rotation. Newer
      formats like 8mm, Hi-8mm and MiniDV use signals recorded into the helical tracks
      themselves, and have no separate control track. While early VHS recorders sported
      manual tracking controls, modern units discern the correct setting automatically,
      taking a few seconds to find the best alignment. Later formats without control tracks
      lock up much faster. However it’s done, the servo must center the rotating heads over
      those tracks for playback to occur, and servo problems are frequent causes of helical
      recorder failures.
           The mechanical sections of VCRs are much like those of any tape recorder, except
      for the necessity of pulling the tape out of the cassette and wrapping it halfway
      around the head drum. This looks easy, but doing it reliably without damaging the
      tape was one of the greatest obstacles in the development of video cassette machines.
      Loading problems are also frequent repair issues, especially in camcorders, with their
      tiny mechanisms that get tossed around in normal use.
           The rest of a VCR is a TV receiver, with a tuner, audio and video sections. Early
      machines recorded audio along the tape’s upper edge in linear fashion, like any
      analog audio recorder. The hi-fi system added frequency-modulated carriers for audio,
      recording them with rotating heads on the drum, along with the video tracks. All VHS
      hi-fi machines still record a monaural, linear audio track for compatibility with non-
      hi-fi units, but later formats never included one.
           The rest of a camcorder is a video camera, typically with a motorized zoom lens,
      autofocus and all kinds of signal processing. Modern camcorders, both analog and
      digital, use digital signal processing in their camera sections, eliminating all the trimpots
      and wads of circuitry used in older analog designs.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 265

  What Can Go Wrong
  Most VCR and camcorder problems result from mechanical issues with the loading
  mechanisms and tape drives. Clogged video heads are common, and cleaning them
  requires care and a gentle, steady hand.
       The tape has to be taken in, properly seated and threaded around the head drum.
  Broken nylon gears are typical in VHS machines. Bent cassette carriages and loading
  arms cause a lot of camcorder failures because the parts are small, thin and easily
  deformed.
       The tape path gets dirty from tape oxide residue and periodically must be cleaned.
  The tension arm with its felt band around the supply spindle, from which the tape is
  fed, often gets bent just enough to misalign the tape path around the heads. That path
  is critical and tough to realign.
       Video heads wear out, making proper tracking increasingly difficult and unreliable.
  In analog recorders, the drum spins at 1800 RPM. In a MiniDV machine, it zips along
  at 9000 RPM. So, for each second of operation, those heads are seeing a lot of tape! It’s
  a wonder they last as long as they do. It takes years of frequent use to wear out VHS
  heads and at least a couple to grind down MiniDV heads.
       The entire machine’s operations are directed by a microprocessor with many
  inputs. Various parts of the loading mechanism report back to the micro via leaf
  switch, mode switch or optical sensor so it can move them in the proper sequence.
  The micro also keeps tabs on whether the spindles are moving and if the head drum
  and capstan motor are rotating at proper speed. A fault in any of these areas can
  trigger a shutdown. Those protections are a legacy of the early videotape years, when
  recorders, video heads and even the cassettes were quite expensive.


  Is It Worth It?
  VCRs are obsolete and cheap, but you might try to save one because you have tapes
  you want to continue to be able to view. Perhaps you even have a Betamax and some
  priceless home movies from long ago you need to dub to a DVD recorder. Camcorders
  range from throwaways to very expensive pro or semipro machines, so some of them
  may be more worth the effort to repair.
      If the heads or other mechanical parts are worn out or broken, forget it, at least
  with VCRs and low-end camcorders. As with CD and DVD players, sources for video
  recorder parts are gone because they’d cost more than the machines they fit, if you
  consider the price of labor to change them. Service shops can still get camcorder
  parts, but it’s unlikely you’ll have access to them.
      Heads are easy enough to clean, and tape path realignment of analog recorders,
  while something of an art form, can be achieved with a known good tape, a steady
  hand and an oscilloscope. Alignment of a pocket-sized digital camcorder’s path is
  pretty nasty, because it’s unlikely you can get to the required signal’s test point
  without specialized factory equipment. I’ve had some luck doing it by trial and error,
  carefully noting the extremes at which picture blocks appeared and then setting the
  tape guides to the middle of the range.
266   How to Diagnose and Fix Everything Electronic

      The Dangers Within
      The rotating head drum is easily damaged, and it can cut you if you contact its edge
      while it’s whizzing around at full speed. Beyond that and keeping your fingers out
      of the way of the loading gears and such, there isn’t much to worry about besides
      the usual power supply stuff. If you’re not working on the supply, cover it to avoid
      inadvertently touching hazardous voltages.


      How to Fix One
      Let’s look at some of the most common problems and how to solve them.

      Stuck Tapes
      If a tape is stuck in the machine, don’t tear it out or you’ll do more damage than you
      were trying to fix in the first place. Stuck tapes mean either a power supply problem
      or a mechanical failure. Most of the time, the rubber wheel driving the spindles has
      dried or worn out. Sometimes that results in a tape spill, and loops of tape can get
      wound around the guides and loading arm. If you rip out the tape, you’ll probably
      bend or break those parts, and that’s the end of the recorder.
            Many machines use a belt to drive the wormgears that raise and lower the
      cassette carriage. Some also use one to move the loading arms that pull the tape from
      the cassette and wrap it around the head drum. When the belt stretches with age, the
      mechanism gets stuck and the micro goes into protection mode, shutting down the
      machine. VCR belts and wheels, once readily available, are fast disappearing. You may
      be able to retrofit a part from another machine. It’s tempting to try a rubber band, but
      those stretch too much to be useful. The loading functions aren’t so exacting, though,
      that a belt close in size but not a perfect fit might not work.
            Take a look at Figure 14-5. Before trying to remove a tape, check the position of
      the loading arms. Are they retracted, with the tape not protruding from the cassette
      shell? That’s the best-case scenario. If you’re lucky enough to find that, hunt around
      the sides of the cassette carriage for the loading motor and wormgear. Look for a belt
      from the motor to the first gear. With all power disconnected, try turning the gear by
      hand. If it won’t turn one way, try the other. In one direction, it should begin to eject
      the tape. Keep turning it until the cassette can be removed normally.
            If the loading mechanism is engaged, with the arms out in the threaded position
      or any position past fully retracted, you have a bigger problem. Sometimes the loading
      arms are properly retracted but the tape is out of the cassette anyway, because the
      spindles didn’t turn to take it up while it was being unthreaded.
            Either way, the problem is the same. While you might be able to remove the
      cassette with the procedure I just described, it’s unlikely you’ll get it out without
      destroying the tape. That’s no biggie if it’s just a recording of some old TV show, but
      if it’s your only record of little Jenny’s first birthday party, you don’t want to lose it!
      Once videotape is creased or stretched, there is no way to flatten it back out and get a
      picture off of it. And if the damage is severe, running the bad spot through a machine
Chapter 14         Aces Up Your Sleeve: Tips and Tricks for Specific Products 267

                                         Pinch roller in
                                         raised position     Audio/control head       Capstan




    Head drum

    Erase head


    Entry guide,
    retracted

   Tension arm,
   retracted




         Felt-lined tension band    Reel drive idler wheel    Exit guide, retracted


                      Figure 14-5     VHS transport mechanism



  later on may clog the heads every time you try it, making it hard to play the rest of
  the recording.
       If you don’t care about the tape, just cut it away and pull the shreds out of the
  mechanism. Then use the manual gear-turning procedure to eject the cassette. If the
  tape does matter, it’s worth trying to remove it intact.
       If things got stuck with the tape threaded, it may already be damaged, but at
  least you can try to avoid causing further carnage. Skin oils ruin videotape, so don’t
  touch it with your bare hands. Put on a pair of fresh disposable rubber gloves, being
  sure to use a type that is not covered in talcum powder or lubricant of any sort. Try
  to extricate the tape from the mechanism as gently as you can. If there’s grease along
  the bottom of moving parts, where they contact the chassis, be especially careful
  not to let the tape touch it. Playing a greasy tape can wreck a video recorder, and
  cleaning the grease from the tape is pretty much impossible without causing serious
  damage to its recorded contents.
       During play, the tape is held against the rotating capstan shaft by a rubber
  roller called the pinch roller. Mechanisms that lower it from above make removal of
  tapes without damage especially difficult if the roller is in its lowered, ready-to-play
268   How to Diagnose and Fix Everything Electronic

      position. To get a tape out of one of those requires manually turning the loading motor
      assembly’s first driven gear by hand, just as with the cassette carriage motor, until the
      loading mechanism unthreads and retracts. The pinch roller will rise, freeing the tape.
      Look for the loading motor on the underside of the tape transport chassis. I’ve seen a
      few on top, but not many.
           Once you’ve extricated the tape from the mechanism, you need to turn either of
      the spindles to take it up before you attempt to eject the tape. It’s a good-old catch 22:
      you can’t reach them with the cassette in place, and you can’t remove the cassette
      until you turn them. Place the transport on its side. On the back you’ll see a pulley
      positioned between where the two spindles are on the top side. Usually, it’s driven
      by a long belt from the capstan motor, though some fancy models use a separate
      motor to turn it. Try to turn it by hand, watching the cassette’s reels to see when
      one is turning the right way to take up the tape. The supply reel, on the left, should
      turn counterclockwise. If the takeup reel, on the right, turns instead, it should go
      clockwise. Turn the pulley while carefully guiding the tape with your other hand,
      keeping it away from grease and from getting caught on the mechanism. When it’s
      fully wound into the cassette, eject it by turning the carriage motor’s gear.
           If the tape is damaged, which is likely, avoid playing that spot on another machine,
      lest you wind up with another repair job on your hands! It’s best to start playback after
      the damaged area.

      Why Did It Happen? Once you’ve gotten the tape out, check for why it got stuck in
      the first place. The cassette itself will not cause this problem unless it has a label on it
      that peels off and jams the machine. Most of the time, stuck tapes are the recorder’s fault.
           Typically, the rubber wheel driving the spindles has lost friction. If that long belt on
      the underside has stretched or gotten dirty enough to slip, it’ll cause the same problem.
      The telltale symptom is that the mechanism has properly retracted but the tape is still
      out of the cassette. Machines with bad belts or wheels often have trouble rewinding, too.
      Although replacement parts are hard to find these days, it may be possible to save the
      old one. Try cleaning it with a swap moistened with a small amount of naphtha. Don’t
      saturate the swab! Just a drop or two will do it. Also clean the mating surfaces of both
      spindles. Let everything dry for a minute or so, and then turn the pulley on the other
      side of the transport while holding whichever spindle tries to turn. See if there’s a good
      grip. Turn the pulley the other way and test the other spindle in the same fashion. If the
      repair is successful, don’t expect it to last for years. Once the rubber wears out, there’s no
      keeping it alive for a long time. It might see you through dubbing some tapes, though.

      Other Loading Problems
      The accepting, loading, unloading and ejecting of tapes are coordinated by the unit’s
      microprocessor. Each process requires several steps, and the micro has to know where
      the mechanical parts are to perform the required sequence successfully. Buried
      somewhere in the mechanism, usually on the underside, is a mode switch with multiple
      contacts that connect when various parts of the mechanism reach their destinations.
      Most mode switches are rotary, with fingers that rub against a set of printed-circuit
      contacts. Some are slide switches. Look for a bunch of wires or a ribbon cable going to
      the switch.
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 269

       Dirty or oxidized mode switch contacts will seriously confuse the machine,
  causing all manner of odd mechanical behavior, from random shutdowns to stuck
  loading arms and out-of-sequence movements that ruin tapes. A little contact cleaner
  spray into the switch often does wonders. Just be careful not to let it get on the rubber
  parts or it will lubricate them too much for proper traction, and you’ll have to clean
  it off. After spraying, run the mechanism through its loading and unloading paces at
  least half a dozen times to help the spray clear the dirt.

  Cleaning
  Any time you service a tape recorder, audio or video, clean the tape path. A dirty path
  will cause symptoms ranging from snowy playback to none at all. Even if playback is
  okay, clean the machine anyway. Helical recorders are especially prone to problems
  from tape oxide and other dirt because the alignment of the tape with the head drum
  is critical down to absurdly small tolerances. As gunk builds up along the drum’s track,
  the lower edge on which the tape rides, it forces the tape slightly upward, disturbing
  the alignment and causing mistracking. Also, the contact surface of the video heads
  themselves is very small, and it doesn’t take much to come between it and the tape.
  Any loss of contact results in nearly complete loss of signal.
       To clean the tape path, you’ll need some swabs, isopropyl alcohol and a sheet of
  white printer paper. Dip a swab in alcohol and clean the loading guides that pull the
  tape out of the cassette and around the head drum. Pay extra attention to the ends
  against which the edges of the tape rub. Those get the dirtiest and are also the most
  alignment-critical. Then clean the tension arm and the erase head, on the left. Next,
  clean the stationary audio/control head on the right, the capstan and the pinch roller.
  Once you’ve cleaned the roller, throw away that swab.
       That’s the easy stuff. The head drum is the most delicate part of the machine,
  and cleaning it without doing damage to the heads, also called the head tips, requires
  extra care. Turn it slowly from above while you look at the slit where the rotating
  section meets the stationary base. As it turns, you’ll see a small, rectangular-ish hole
  pass by. Sticking ever so slightly out of the hole is a video head. See Figure 14-6. The
  head is very narrow. It’s made of a strong but brittle ferrite material that can endure
  thousands of hours of high-speed rubbing against the tape. It cannot withstand much
  up-and-down pressure at all! If you push up or down on it, you will snap it off, and
  that’s the end of the recorder. You’ll find anywhere from two to six holes with heads,
  depending on how fancy a model the recorder happens to be.
       To clean the drum, first dip a fresh swab in alcohol and clean the edge of the
  track, which runs diagonally along the bottom, from its highest point at the left to its
  lowest at the right. This is where the tape rides, and it needs to be really clean for
  proper tracking to be assured. Then clean everything between the track and the slit,
  carefully avoiding the video heads. (Rotate them out of the way as you go.) Once that’s
  done, dip another swab and clean the rotating drum everywhere except very near the
  video heads or the heads themselves. Avoid those babies! Clean the drum by holding
  the swab and turning the drum against it from above, being careful not to get finger
  oils on the outer surface, which contacts the tape.
270   How to Diagnose and Fix Everything Electronic




                       Figure 14-6     Close-up of video head tip



           Now comes the fun part. Take the printer paper and fold it in half. Wet an inch or
      two of one side with alcohol. If alcohol is dripping from it, let that drip off, and blot
      the paper with another piece. Turn the drum so no video heads face you. Press the
      wet area of the paper against the drum, but not hard. Make sure some of that pressure
      is at the slit. Now turn the drum slowly so the video heads will rub against the paper
      as they pass by. Rotate the drum several times, being careful not to put pressure on
      the heads in an up or down direction. Remove the paper and let the drum dry. You
      should now have some black streaks on the paper and one clean VCR!

      Tracking Problems
      If the tape doesn’t track accurately, playback signal will be lost through part of the
      head’s sweep across the tape, and you’ll see snow somewhere in the picture, or
      the image will jitter vertically. There are several varieties of tracking maladies. If the
      machine won’t track a tape it recorded, there’s a loss of head contact from worn-out
      heads, dirt, misalignment of the entry and exit guides controlling the tape position on
      the drum or a lack of proper tape tension. Check the tape tension by gently moving
      the tension arm on the left. As you press it toward the left, the buzzing of the heads
      against the tape should increase significantly. As you push it right, the buzzing should
      get quiet. If tension is too low, there won’t be much difference between its normal
      position and when you push it toward the right. The arm connects to a band wrapped
      around the left spindle. Between the spindle and the band is a coating of felt. The felt
      may be worn, or the mating surface on the spindle might be dirty. Clean the spindle’s
      surface with alcohol and check again.
           Sometimes the tension arm gets bent upward a little from pulling against the
      tape, so the tape doesn’t sit properly on the guides. Gently bending it back may
Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 271

  solve the problem. The tension arm should sit straight up and down. Bend it only if
  it is obviously tilted. Even then, go very carefully, because it’s easy to wind up with
  serious alignment problems if you get it out of whack.
        If tapes made on the machine look fine but it won’t play those recorded on other
  VCRs, there are two possible reasons: one of the guides may be off, or the position of
  the audio/control head may be incorrect. To test, play a tape recorded at the slowest
  speed on a VCR whose alignment you trust. Run the tracking through its range using
  the buttons on the remote (unless it’s a really old VCR, in which case it may have
  a knob on the front panel). As the tracking shifts, snow will appear. If it’s mostly at
  the top or bottom of the picture, and it moves up or down a lot when you change the
  tracking, it’s a guide problem. If it appears across the entire picture at once, or nearly
  so, the audio/control head’s position is off.
        Adjusting the audio/control (a/c) head is easy. Set the machine’s tracking control
  to the center of its range. On one side of the a/c head, you’ll see a cone-shaped screw.
  Turning it will move the head left or right without changing its tilt. Adjust the screw
  just a little. If the picture gets better, turn it some more, until the image looks good. If it
  gets worse, turn it the other way. Try not to turn it more than a few degrees either way.
        Run the tracking through its range again and see if picture noise appears at
  approximately equal distance away from the center position. Adjust the audio/control
  head until that’s the case. You should wind up with optimum tracking at or very near
  the center of the machine’s tracking adjustment range.
        Adjusting the guides is a much more involved affair. To set them, you need a
  tape recorded at the lowest speed on a machine you trust, as before. Put it in and set
  the tracking for the least snowy picture, even if one end has noise. Try to minimize
  the overall noise. It’s better to have a strong picture with bad noise at one end than a
  slightly noisy image across the whole screen.
        Warm up the ol’ oscilloscope and set it for 200 mv/div and a sweep rate of 5 ms.
  Look for two cables coming out of the head drum, one from the top and one from the
  back. One of them will lead to a shielded area. It may be a separate metal box, or it
  may just be a shield over the circuit board. That’s the video head preamp. On it or
  nearby will be some test points on a connector that has nothing inserted into it. Using
  the metal chassis as ground, scope the pins and you’ll find a signal that looks like
  the upper waveform in Figure 14-7. This is the RF envelope, an amplified rendition of
  what’s actually coming off the heads. Connect your probe to it, being careful not to
  short adjacent pins with the clip. Scope triggering will be unstable, so the waveform
  will jitter from side to side.
        To get a stationary trace, set the scope’s second channel to 5 volts/div and the trigger
  to channel 2. Enable chop mode and scope some of the other pins on that connector
  until you find a square wave. Adjust the trigger level control for a stable sweep. You
  should now be able to see a rock-solid envelope, with the triggering square wave
  underneath. The envelope contains the signals from both heads, one after the other, and
  you can view the interruption between them at the point the square wave changes states.
        The start of the head sweep, and thus of the envelope waveform, is on the left.
  In the recorder, that corresponds to the entry guide just to the left of the drum. If the
  envelope is lower at the start of each sweep, that’s the misaligned guide. Before you
  adjust it, take a good look at how the tape rides on it. If it’s high, not touching the
272   How to Diagnose and Fix Everything Electronic




                              A head         B head        A head




                                           Transitions




                        Figure 14-7     RF envelope

      guide at the bottom, the tension arm may be bent upward a little bit, and you should
      gently bend it straight before trying to align the guide. Watch how the tape rides on
      the guide to get this correct. See Figure 14-8.
          If the waveform is low on the other side, the problem is the exit guide, on the right.
      To align either guide, you need to unthread the tape and slightly open the setscrew at

                                            Entry guide




                                                                                    Tension arm




          Figure 14-8      Entry side with tension arm
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 273

  the bottom of the guide, if there is one. Don’t make it loose; you need some friction
  there. If there’s no setscrew, you can proceed without worrying about it. Hit play.
  When the tape is loaded and playback begins, turn the split screw adjustment at the
  top of the guide to lower or raise it. Tiny amounts suffice here; it’s unlikely you’ll need
  to turn it more than a few degrees.
        The correct adjustment tool is a special split screwdriver made for the purpose.
  Assuming you don’t have one, you can use a normal screwdriver on one side, but be
  very careful that it doesn’t slip off and whack into the spinning head drum. If it does,
  you’ll probably break off a video head.
        The two guides interact with each other and also with the track at the bottom of
  the drum. If the center of the waveform dips while the two ends do not, at least one
  of the guides is too low. Try to get the waveform as flat as possible. If there’s some
  dipping at the ends, that’s okay as long as it’s not severe. Video recording uses FM,
  so no quality of signal is lost as long as the waveform stays above a certain threshold.
  It’ll wobble around slightly, because tape motion isn’t perfect and we’re dealing with
  insanely tight tolerances here.
        As you adjust the guides, listen to the buzz of the heads hitting the tape. When the
  tape rides too low, the buzz increases quite a bit. The correct setting will be where it
  just starts to increase and sounds as stable as possible.
        Unless the heads are badly worn, most of the adjustment issues will be at the
  start and end of the envelope, not in the middle. In particular, the entry guide side of
  the drum tends to have problems. To get a nice, close-up view of that transition, use
  delayed sweep to magnify just that portion of the waveform. Be sure to zoom back out
  afterward and check the entire envelope. Don’t expect perfection; some amplitude
  variation between heads is normal. And the transitions will wobble up and down
  slightly. See Figure 14-9.




                   A head at exit guide   B head at entry guide




                    Figure 14-9       Magnified view of head transition
274   How to Diagnose and Fix Everything Electronic

          Adjusting those guides is an art form that takes a lot of experience to do well.
      Don’t be surprised if you wind up going back and forth from guide to guide. The first
      few times I did this procedure were very frustrating. Stay with it, and you’ll get the
      hang of it. If nothing works, and especially if the center of the envelope dips when
      the ends are correct, and you can’t adjust your way out of that problem, the heads are
      probably worn out.

      Digital Recorders
      The ideas are the same, but the digital signal looks very different. You probably won’t
      even find it in most camcorders. Your best bet is to take a tape from when the camera
      was new and use it as a reference, unless you have another digital camcorder whose
      alignment you trust. The tiny mechanisms in MiniDV camcorders bend easily, so
      check that the cassette is seated properly and the tension arm isn’t bent. If necessary,
      carefully adjust the guides, watching for picture breakup instead of snow. Take note
      of the guide settings where it occurs and set them in the middle of the range. Use
      your ears to be sure the tape isn’t riding too low. Also, if it’s too high, the buzz will
      get much quieter. Find that magic spot, and you should have a good picture. Digitals
      use error correction and are pretty tolerant of data loss, so some misalignment gets
      masked, even though the drum and tape are smaller and the tolerances are tighter.
      I’ve lined up a few digital camcorders using this technique, and they all work fine and
      interchange with each other well, even in LP mode.

      Servo Problems
      Helical recorders control both head and capstan rotation with servos. Mounted on
      each motor are PG coils, which are pulse generators. The coils pick up a field from
      a magnet in the rotating section, generating a pulse each time the magnet goes by.
      This is fed to one input of the servo. The other input is a reference signal. The servo’s
      job is to time-align the two by adjusting the motor speed and phase to match the
      reference signal’s timing.
           If a servo isn’t working, the machine will mute, with no picture or sound on
      playback. You can verify a servo problem by scoping the RF envelope. If it has wild
      amplitude swings running through it randomly, the heads are not synced to the tracks
      because one of the servos isn’t working.
           Take a look at the tape reels. If they’re moving fast even though the tape was
      recorded at slow speed, the capstan servo is out. If tape speed seems okay, look at
      the head drum rotation under a standard fluorescent lamp (not the spiral type). You
      should see a slowly creeping, almost stable pattern with a normal rate of rotation.
      Listen to it too. If it’s wildly off, the head servo is not working.
           Servo operation is complex, but it’s been reduced to a couple of chips. Many
      servo problems are caused by loss of signal from the PG coils or one of the reference
      signals. The one most likely to be troublesome is the control track pulse from the
      audio/control head. The control track is recorded along the bottom edge of the tape.
      If you look at the face of the head, you’ll see the upper and lower recording surfaces.
      Make sure the lower one is clean. Sometimes wear, misalignment or low tape tension
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 275

  will cause the lower surface to lose contact with the tape, wiping out playback of
  the control track or making it intermittent. The machine will go in and out of servo
  lock, showing a picture and then muting repeatedly, especially near the beginning
  of a tape, when tension is the lowest. (A worn or stretched tape can also cause this
  symptom.) Use a dry swab to press on the tape’s lower edge where it meets the head
  while a tape is playing. If the servo suddenly locks in, there’s your problem.
       One of the screws on the head mounting will control the zenith, or forward-
  backward tilt. You should be able to deduce which one it is. Turn it to pull the top back
  a little, away from the tape. That’ll put more pressure on the bottom and may restore
  servo operation. If you pull it back too far, the linear (non–hi-fi) audio track won’t
  play, because the upper surface will lose tape contact.
       If you really want to delve into the servos further, scope the PG coil pulses and
  follow their cables back to the servo circuits. Beyond that and some poking around
  with the scope, it’s really not worth what it would take to hunt down obscure problems,
  given the value of the machines these days.

  Color Problems
  In analog machines, tape path misalignment can cause color distortion at the top or
  bottom of the picture by twisting or slightly stretching the tape, altering its timing
  characteristics as the heads read the signal. This usually occurs on the left side of the
  drum. You should be able to recognize such dimensional distortion visually and adjust
  the guide or correct the tension arm. Be sure to check the envelope after changing the
  tape path. You can do a quick-and-dirty check with the tracking control, watching for a
  reasonably even appearance of snow across the entire picture as you set the tracking
  away from its optimal point.
      Color and luminance are processed separately in an analog video recorder and
  then recombined upon playback. The color subcarrier, 3.579545 MHz in the United
  States, is converted down to a much lower frequency before being fed to the video
  heads. On playback, it goes through a rather convoluted system of two PLLs that
  corrects the timing errors before it’s mixed back into the monochrome part of the
  signal. If the machine plays back only in black and white, first be sure the tape you’re
  using has properly recorded color on it. Then look for the 3.579545-MHz crystal on the
  board. Sometimes it’s labeled 3.58. Make sure it’s oscillating. You should see another
  couple of crystals near that one, so check their operation too. One of them is used
  only while recording. If you see no signal on it, put the machine into record, using
  another tape to avoid wiping out your test tape, and see if that crystal runs.

  Audio Problems
  The linear, non–hi-fi track is recorded along the top edge of the tape exactly the same
  way as in any audio tape recorder. The hi-fi tracks use FM subcarriers and are recorded
  right on top of the video tracks by an extra set of heads in a VHS machine. In Beta and
  8 mm formats, the signal is mixed with the video carrier and recorded by the same
  heads that lay down the video tracks. In MiniDV camcorders, the audio is encoded into
  the bitstream along with the video data and recorded by the same heads as well.
276   How to Diagnose and Fix Everything Electronic

           Loss or breaking up of hi-fi audio tracks is usually due to dirty heads or mistracking.
      Significant mistracking can occur around the vertical sync area of the signal, off the
      screen. Usually that’ll cause vertical jitter, but sometimes it’s placed just far enough
      from the vertical sync to avoid affecting the image. It’ll still make a mess out of the
      hi-fi audio, though. Check the RF envelope. If it looks okay, try cleaning the video heads
      again; one of the hi-fi audio heads may still be dirty.
           Dropping out of MiniDV digital audio usually means misalignment or dirt in the
      tape path. The format covers missing video blocks pretty well with blocks from the
      last image, so it may be masking loss of data, but the audio will still mute.



Video Projectors
      LCD and DLP video projectors are quite popular, and each type has its characteristic
      failures. Let’s look at projectors and how to work on them.


      How They Work
      Front projectors and modern, non-CRT rear projection TVs use the same technologies,
      with a bright lamp, a “microdisplay” device that forms the image, and a series of lenses
      to magnify the results. In LCD projectors, there are three small LCD panels, one for
      each primary color of red, green and blue. Light from a very high-intensity arc lamp is
      filtered to remove ultraviolet energy and then split into three beams, with color filters
      for each color. Each beam illuminates its own panel. The resulting three images are
      recombined with a prism and focused on the screen with the projection lens.
           DLP projectors have no LCD panels. Instead, they use a special chip called a Digital
      Light Processor, invented and manufactured by the Texas Instruments Corporation. On
      the surface of the DLP chip is a matrix array of microscopic mirrors, each separately
      addressable. Depending on the resolution, there may be hundreds of thousands to a few
      million mirrors. Feeding power to a mirror makes it flex, deflecting the light at an angle
      and reducing the amount reflected straight toward the lens. The result is a projected
      video image of high contrast.
           That’d be all there is to it, except for one small detail: color. DLP chips are
      expensive, and they require a fair amount of circuitry to drive them. Some very
      pricey, professional-level projectors have three DLPs, combining their outputs like
      LCD units do. The home units you’re likely to service, though, have only one chip and
      accomplish color projection by rapidly flashing the three color images in sequence
      with a high-speed rotating color wheel of red, green and blue segments between
      the lamp and the DLP chip. Your visual system, which can’t keep up with anything
      coming at it that quickly, combines them into one full-color image. DLPs can move
      their mirrors much faster than is required for normal video rates, so it’s possible for
      them to flash two or even three complete sets of tri-color images in the time span
      of one frame. When the specs say the unit has a 2X or a 3X color wheel, each frame
      of video is being flashed at that rate, compared to a normal video frame, with three
Chapter 14     Aces Up Your Sleeve: Tips and Tricks for Specific Products 277

  flashes of color each time. So, a 3X-rate projector flashes nine images in the time it
  would take a CRT to scan one frame.
       The increased rate helps diminish the “rainbow effect,” an annoying consequence
  of the single-chip, frame-sequential color projection method that occurs when the
  viewer’s eyes move. Especially in darker images with bright points of light, like night
  scenes with streetlamps, the visual trail left by the bright spot can break up into its
  component colors as the eyes change position, because each color frame strikes the
  retina in a slightly different spot, so they don’t blend together. Some people find the
  effect very distracting, so manufacturers keep speeding up the color wheels and frame
  rates to minimize the time between projection of the different colors, keeping them
  closer together in the moving eyes of the viewer.


  What Can Go Wrong
  There’s lots to malfunction here. The most troublesome elements in a projector are
  the very expensive lamp and the circuitry powering it. The brightness required is so
  extreme that only a high-pressure mercury vapor arc lamp will do the job. Operating
  an arc lamp is not as simple as just applying power. First, it has to be “struck,” or
  started, by applying a fairly high voltage until conduction across the arc is achieved.
  Then, once current starts flowing, the mercury inside vaporizes and makes the lamp
  conduct much more readily, with lower resistance. The voltage must then be reduced
  to typically less than 100 volts. The circuitry driving the lamp is called the ballast,
  though it’s much more complicated than the simple ballast that starts an old-fashioned
  fluorescent lamp.
       Wait, there’s more. Arc lamps exhibit some pretty odd behaviors. As they age, they
  tend to develop bad spots on their electrodes, increasing the resistance of the most
  direct path across the arc. Because the vapor conducts, other paths arise, and the arc
  can jump around, causing flickering of the light. To combat this annoying malady,
  the ballast may adjust the operating voltage or add pulses to keep the lamp at its best.
  Even with all this effort, lamps go bad, they go dim, they fail to strike, and now and
  then they explode violently.
       The second most failure-prone part differs between LCDs and DLPs. In LCDs,
  the polarizers, sheets of plastic film in front of each panel, get burned by the residual
  ultraviolet output of the lamp, even after its light has passed through the ultraviolet
  filter. The blue polarizer, in particular, tends to burn, resulting in a yellowed image or
  splotches of yellow.
       In DLPs, the color wheel, whizzing around so fast, often experiences motor failure
  or catastrophic mechanical failure. Because of lamp heat, color wheels are made of
  glass, not plastic, and some are assembled with nothing more than glue! In time,
  the glue degrades, also from lamp heat and ultraviolet, and the delicate red, blue
  and green segments fly off, smashing against the inside of the projector’s case and
  shattering into a million pieces.
       DLPs also use a light guidance arrangement quite different from that in LCDs.
  LCD panels are considerably larger than DLP chips, and the lamp’s output is spread
  over enough area to illuminate them fully. That’s easier than the DLP scheme, in
278   How to Diagnose and Fix Everything Electronic

      which the light has to be formed into a small beam. To do that, DLPs use a mirror
      tunnel, sometimes called a light tunnel, made of four mirrors arranged to form a
      rectangular channel. Like the color wheels, the mirrors may be glued together, and
      the glue can fail, collapsing the tunnel.
          Cooling the lamp is a critical function in all projectors, so they all have fans
      blowing air through the lamp housings. Most projectors have multiple fans. LCD
      units usually have one just to cool the panels, because, as blocking elements, they
      absorb a lot of heat from the lamp. DLPs, which reflect light instead of absorbing it,
      don’t overheat their imaging chips, but they sometimes use fans to cool the rest of the
      optical chain, along with the lamp fan. Some projectors have power supply fans as well.
          A failed fan, especially if it’s the lamp fan, will cause the projector to overheat and
      shut down. It takes a few minutes for the thermal sensor to heat up enough to cause
      shutdown, so the projector will run for a short time before it dies.
          Most projectors have dust filters on the lamp housings, and they get clogged
      with room dust to the point that airflow is severely restricted, triggering an overheat
      shutdown.
          And, of course, projectors suffer from the usual power supply issues, especially
      bad capacitors. The units are remote-controlled, so at least part of the power supply
      runs all the time, even in standby. After a few years, a cap or two is shot, and the
      projector stops turning on.


      Is It Worth It?
      An expired lamp might seem like an obviously worthwhile repair, but the lamps cost
      so much—from around $100 to more than $400—that you must consider whether the
      rest of the projector will survive long enough to use up a new lamp. Those hot lamps
      put tremendous stress on the other optical components, and many projectors are
      designed to last about as long as one lamp. It’s no fun to spend two-thirds the cost of a
      new projector for a replacement lamp, only to have a polarizer or a color wheel go bad
      100 hours later. Some expensive, pro-level projectors are built to last through several
      lamp replacements, but the relatively inexpensive home units are not.
           Burned LCD polarizers are pretty much a dead end unless you can scare up a parts
      unit. It’s almost always the blue polarizer that goes, so a unit old enough to be cut up for
      parts probably has the same bad polarizer and will be of no use. Manufacturers don’t sell
      the polarizers separately; they want you to buy the entire light engine (optical system)
      or replace the projector. Believe me, you do not want to spend what a new light engine
      would cost.
           Color wheel costs vary greatly among manufacturers. The wheels for some
      rear-projection DLP TVs can be had for $50, while those for some front projectors
      cost an eye-popping $500. There’s no basic difference between the parts; it’s all
      a matter of marketing and volume. Parts for TVs, including lamps, are generally
      lower than those for front projectors.
           Light tunnels can often be repaired for nothing with some epoxy and a steady hand.
Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 279

  The Dangers Within
  The lamp is not your friend! If you look directly into it while it’s running, even
  momentarily, I hope you like dogs, because you’re probably going to need one. The
  brightness is higher than anything the human eye can withstand. There’s a fair amount
  of ultraviolet, too, which is very damaging, even after most of it is absorbed by the
  lamp housing’s UV filter. Seriously, don’t ever look directly into a running projector!
       The lamp gets hot enough to burn you badly, too. After it’s been operating, let it
  cool down quite awhile before going near it. When it’s hot, the glass is more fragile
  as well. The actual lamp envelope is only about the size of two pencil erasers, but it’s
  under tremendous pressure. I’ve seen one explode, and it’s not pretty. They go off like
  a shot, and a fine mist of glass particles tinged with mercury gets ejected from the
  projector’s fan vent; you wouldn’t want your eyes in the vicinity.
       The voltages used to strike and drive the lamp are hazardous. Figure around 1 KV
  for striking and 80–100 volts during normal operation. Don’t go poking around with
  your scope in the lamp supply (ballast), especially while the lamp is striking.
       A DLP’s rotating color wheel could cut you if you contact its outer edge while it’s
  spinning, but it’s more likely you’d destroy it.


  How to Fix One
  See Figure 14-10 for a view of the optical path, or light engine, in a typical DLP projector.
  Most problems are found there or very nearby. The three most common failures are
  no operation at all, no lamp strike, and overheating with subsequent shutdown. If the
  unit won’t turn on at all, suspect the usual power supply issues. Projectors and TVs
  spend virtually their entire existences plugged in, waiting for a remote-control signal,
  so bulging power supply capacitors are pretty much a foregone conclusion eventually.
      One difficulty in servicing projectors is that restarting a hot lamp damages it
  badly. So, once you turn the unit on, you don’t want to turn it back off, take a few
  measurements or check a few parts, and then fire it right back up again. Always let
  the lamp cool before restriking it. That can take a half-hour or so.

  Lamp Problems
  If the projector powers up but blinks a warning light on the control panel, the lamp or
  its ballast may be bad. A sensing circuit checks for current draw through the lamp; that’s
  how the thing knows the lamp has struck and it’s time to reduce the striking voltage to
  its normal running level. If the lamp won’t strike, the warning light is as far as it’ll go.
       You can’t check the lamp for continuity with a meter; at room temperature, the
  bulb is an open circuit until it has 1 KV or so applied across it. Assess the lamp’s
  condition visually. Being certain it is cool, remove the lamp. Unless the bulb itself is
  completely enclosed by its housing, it’s a good idea to wear goggles, just in case you
  bump it or drop a tool on it, because it could explode in your face.
       Some TVs use bare lamps, with nothing enclosing them. That little stalk protruding
  from the front is the end of the actual bulb, and it’s fragile! At the bottom of it, nearest
  the back of the reflector, is the high-pressure envelope. The lamps in most projectors
280   How to Diagnose and Fix Everything Electronic

                Lamp compartment     Light tunnel   Fan   DLP chip   Light condenser




       Color wheel




                                                             Projection lens   Mirror


                         Figure 14-10      DLP optical path


      are enclosed in a housing with a UV filter in front, so they’re a little safer, but you
      could still get showered with glass blown out the sides of the housing, through the dust
      filters.
           It’s important never to touch the bare lamp or the UV filter, because skin oils will
      make them crack when they get up to operating temperature. A cracked UV filter may
      leak UV radiation, and a cracked bulb…well, you know what will happen. Kaboom!
           Look at the envelope head-on, slightly from the side. You’re looking down the
      focal point of a parabolic reflector, so it’s hard to see the arc gap, but if you look a bit
      off-axis, you can see it. It’ll look greatly magnified by the reflector, and that’s helpful.
      If the envelope’s glass looks clear and clean, the lamp doesn’t have lots of hours on it
      and is probably good. If it looks charred, it’s an old lamp and may be shot. Examine it
      carefully, and you can also see the actual electrodes and their condition.
           Almost all projectors have a time counter in their menus telling you how many
      hours are on the lamp. Without a working lamp, of course, you can’t see it!

      Ballast Problems
      The ballast is really a pretty fancy power supply of its own. It has to supply the high
      striking voltage and then the lower operating voltage. Operation can require a few
Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 281

  amps, for around 150–250 watts of lamp power. Output transistors supply it, and they
  can pop. Also, a fair amount of heat is generated in the output stages of some ballasts,
  so check the circuit board for burn marks and degraded solder joints.
      Don’t try scoping the ballast’s output stages. It’s far safer and easier to disconnect
  power and pull and check the output transistors. There may be an onboard fuse, too.
  Be sure to discharge any large electrolytics before desoldering anything.
      Look for optoisolators in the path between the lamp and the ballast. The output
  of one of them will change state when the lamp successfully strikes, relaying the
  information that it’s time to lower the voltage. If you can’t find that signal, the lamp
  is not being struck, or it’s bad.

  Overheating Problems
  Overheat shutdowns don’t happen instantly; it takes a little while for the heat to
  build up. If the projector runs for five or ten minutes and then quits, it’s probably
  overheating. A lamp very near the end of its life can run excessively hot and cause
  this condition, but most of the time it’s due to lack of airflow over the bulb. Check first
  for blocked dust filters. Some projectors, especially LCDs, have them at the air intakes
  for both the lamp and the LCD assembly. Blockage of either can trip the shutdown.
  DLPs usually have filters right on the lamp housing. Some units are designed to be
  dust-resistant and have no filters. They rarely clog up, but check the air channels at
  the lamp housing just to be sure nothing has gotten in and blocked those.
       There’s a thermal sensor over the lamp. If the fan doesn’t do its job, the lamp
  overheats and the unit shuts down. Never defeat this sensor, even just for a repair test.
  Even if you don’t cause a fire, you’ll probably destroy the lamp, and it could get hot
  enough to burst.
       The fans themselves can fail, but sometimes the problem is the circuitry powering
  them, especially in units with variable-speed fans. Many projectors have normal and
  economy settings for lamp brightness, with greatly increased lamp life at the lower
  setting. To reduce noise, they slow the fans down in the economy mode, using power
  transistors to lower the voltage. Over time, a transistor may fail, and the fan will go
  dead. The fans should always be turning when the projector is on. If one of them isn’t
  spinning, disconnect power and try turning it by hand. If it’s not gummed up with dirt
  or dried-out lubrication, it should move freely.
       The fans are the same types used in desktop computers, and they usually run on
  either 5 or 12 volts when at full speed. Check to see if any voltage is getting to them. It
  may be less than the fan’s specified voltage, but it won’t be a fraction of a volt, and it’ll
  never be zero if things are working. Expect at least a couple of volts. Don’t use circuit
  ground for this test; check directly across the fan’s positive and negative leads. If a fan
  shows voltage across its leads but is not moving, the fan is bad. You can probably find
  a compatible replacement fan from a computer supply house. Just be sure the new
  one moves at least as much air.
       If there’s no voltage, the driving circuit is out. Trace the fan’s wires back to the
  board and look for a small power transistor. Scope to see if it’s getting power supply
  voltage and if that is getting fed to the fan. The transistor may be between the fan’s
  negative terminal and ground, with supply voltage going to the fan’s positive terminal.
282   How to Diagnose and Fix Everything Electronic

      If you see the same voltage on the fan’s negative terminal, the transistor isn’t pulling
      current to ground and is probably open. That’s why you can’t use circuit ground when
      testing the fan; you need to be sure there’s actually a voltage difference across it.

      Light Tunnel Problems
      If the light tunnel collapses, the projector will run but you’ll see a darkened area
      along an edge or even large portions of the image blacked out. Gluing it back together
      is a chore, but the price is right! Carefully disassemble the optical path and remove
      the tunnel’s mirrors. The shape of the tunnel corresponds to the aspect ratio of the
      imaging chip. You should be able to deduce how it went together from the position
      in which you find the pieces, and from the glue remnants. Clean off the old adhesive
      and glue the tunnel back together with epoxy; instant glue will not survive the lamp’s
      heat. Use the good stuff that takes awhile to dry, not the 5-minute quick-set type.
      If you can find high-temperature epoxy, that's your best bet. Let it dry for a night
      or two and then reassemble.

      Color Wheel Problems
      The DLP’s color wheel is driven by a small motor, with a position sensor to tell the video
      circuitry when to flash the correct image for whatever color is in front of the lamp.
      Most of them use a Hall-effect sensor that picks up a magnetic field from a magnet
      embedded into the assembly’s rotor, generating a pulse for each revolution of the wheel.
      Without that signal, the unit’s microprocessor assumes the wheel isn’t turning properly,
      or at all, and stops operation, shutting down the lamp.
           Treat the color wheel gently. It’s delicate, and breaking it means the end of the
      projector unless a petal has come off intact, in which case you can glue it back on.
      When the unit starts up, see if the wheel is turning. It should spin very fast. If it
      seems sluggish or isn’t moving at all, unplug the projector and gently turn the wheel
      by hand. If it doesn’t turn freely, the motor’s lube may have dried out. Often, dried
      lube will cause a horrible screeching sound from the wheel, too. The normal remedy
      is replacement, but the cost can be ridiculous.
           I’ve saved a few screeching color wheels for nothing, though, and the fix has lasted
      a long time. Get some silicone lubricant spray. Don’t use anything else, like oil or other
      kinds of sprays. Remove the wheel and spray into the motor bearings, saturating the
      motor as best you can. You’ll probably be spraying under the color disc itself into the
      top of the motor. Let it soak in for a few minutes, and then dump out what’s left into a
      tissue. Very gently clean excess lubricant off both sides of the color disc with an alcohol-
      soaked swab. Don’t put pressure on that disc or you may snap off the glass petals. Give
      the motor a few careful spins and then reassemble the unit. Sometimes it works.

      Video Processing Problems
      The circuitry driving the LCD panels or the DLP chip is dense and complex. It’s all
      digital and not very serviceable. If you’re losing a color on an LCD, or the image is
      flickering or freezing on a DLP, check for bad cable connections to the boards driving
  Chapter 14      Aces Up Your Sleeve: Tips and Tricks for Specific Products 283

    those imaging devices. Also look for leaking surface-mount electrolytic capacitors.
    The heat of a projector can really shorten their life. If there’s no video at all, look for
    on-board voltage regulators, and scope to see if they’re putting out voltage. Beyond
    that, you’d need a service manual and some serious, high-speed digital test gear
    to make much sense out of the signal processing. These kinds of problems aren’t
    common, though. The vast majority of projector failures can be traced to the power
    supply, the ballast, the lamp and the optical components.

    Blobs in the Image
    These are caused by dust on the LCD panels or the DLP chip. A little carefully applied
    compressed air will take care of them. You may have to open a housing or remove
    the lens to gain access. Be careful not to spray the air too close to the imager, in case
    some of the cold propellant winds up on the optical surface, where it could mar the
    transparency. The actual mirrored surface of a DLP chip is behind a sealed window,
    so there’s no chance of harming the mirrors. LCD panels and their polarizers are more
    easily damaged.



Have at It!
    I hope you’ve enjoyed our extended romp through consumer electronics and its
    repair. Armed with the techniques presented in this book, you should be ready to dig
    in and have some fun! Just always remember to put safety first, and keep in mind
    that, as with any skill, expertise develops over time. If you wreck a product while
    trying to fix it, toss its remains on the parts pile and chalk up the loss to a valuable
    lesson learned. We all lose a few along the way. The more you practice, the better
    you’ll get.
         Ready to rock? Scope on? Soldering iron warmed up? Ignition sequence
    start…T minus 3, 2, 1…blastoff!! Have a grand adventure!
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Glossary

  H    ere are some of the common terms you’ll encounter when working on electronic
       devices. There are plenty more, but you’ll see these often.

  active elements The semiconductors at the heart of a circuit stage which perform
  amplification, switching or other signal processing functions defining the stage’s
  operation. Transistors and IC chips are active elements. Before solid-state electronics,
  the active element was the vacuum tube.

  ADC Analog-to-digital converter. A device that converts analog signals to a digitally
  coded representation.

  alternating current (AC) The polarity periodically reverses direction. Reversals
  may occur at any rate above zero cycles per second, or hertz, up to billions per second.

  amperes (amps) The volume of flow of electric charges per unit of time, or current.
  This corresponds to how much electricity is flowing, independent of how hard, and
  is represented as A or sometimes I. In small-signal circuitry, it is usually specified in
  ma, or milliamps (thousandths of an amp). The polarity of the current will always be
  the same as that of the voltage, as the voltage is what drives the current through the
  circuit, so it has to be going in the same direction.

  amplitude The voltage strength of a signal. For audio, it represents loudness. For
  video, brightness.

  analog A method of representing information by varying a voltage over time in a
  continuous pattern resembling the information, distinctly different from the either/or,
  on/off states of digital representation.

  audio An analog signal representing sound by varying a voltage in the same pattern
  as the original mechanical vibrations of the sound.

  audio-frequency (AF) A signal in the frequency range of 20 Hz to 20 kHz, the
  approximate spectrum of human hearing.

  ballast The circuit used for starting a lamp. Ballasts range from very simple, as with
  old-fashioned fluorescent lamp tubes, to complex, as with the high-pressure arc lamps
  used in video projectors.


                                                                                       285
286   How to Diagnose and Fix Everything Electronic

      bias The DC current or voltage supplied to the input of an amplifying element to
      keep it in its conducting state during the required portion of the signal waveform.

      bipolar A circuit with both positive and negative power supply polarities relative
      to ground.
          Also, the internal construction of an NPN or PNP transistor.

      bus A large connection point used to distribute power or provide ground to multiple
      areas of a circuit. In digital devices, the shared connections used to distribute data to
      multiple points.

      bypass capacitor A capacitor connected to shunt a signal’s AC component to ground
      without having an effect on the signal’s DC component, if there is one.

      capacitance Specified in farads, capacitance is a measure of how much charge can
      be stored on the two plates of a capacitor for a given applied voltage. The more charge
      stored, the more current will pass back out of the capacitor when it is discharged into
      a circuit.
           Numerous factors affect capacitance, including surface area of the capacitor’s
      plates, their distance apart and what separates them. Most capacitors are specified in
      microfarads (µf, or millionths of a farad) or picofarads (pf, or trillionths of a farad).
      In European gear, capacitors specified in nanofarads (nf, or billionths of a farad) are
      sometimes used.

      carrier A signal upon which another, lower-frequency signal is imposed. In radio,
      the high-frequency signal emanating from the transmitter, on which is imposed
      amplitude or frequency modulation of audio information.

      CCFT Cold-cathode fluorescent tube. The type of lamp used in LCD backlights.
      CCFTs have no filaments. They are driven with high-frequency, high-voltage pulses
      applied across the length of the tube, and do not require a ballast.

      chopper The transistor used in a switching power supply to convert incoming
      power into high-frequency pulses that will be fed to the conversion transformer.

      clipping The state of nonlinearity in an amplifier that occurs when an input signal
      attempts to drive its output to voltages greater than those provided by the power
      supply. The amplifier’s output signal can’t swing as far as the input signal demands,
      so it stops at its limits and clips off the tops and bottoms of the waveform, causing
      severe distortion. See linear.

      CMOS Complementary metal oxide semiconductor. This type of integrated circuit
      construction is widely used because of its wide voltage tolerance and low-power
      operation. Once limited to small-signal circuits, CMOS devices now handle serious
      voltage and current, and are found in MOSFET power transistors and output modules.
                                                                    Glossary        287

Some CMOS devices are easily damaged by static electricity because it can punch
holes through the oxide layer, causing an internal short circuit.

complementary Opposite in phase. Also, an amplifier design with separate transistors
operating on opposite halves of the signal waveform. A true complementary design uses
PNP and NPN (opposite-polarity) transistors. One using the same polarity transistors for
both waveform halves is called quasi-complementary. Both schemes are used frequently
in audio power amplifiers, with the two transistors shown one above the other on the
schematic diagram. Complementary amplifiers are called Class AB.

coupling The passing of a signal from one circuit stage to another. Typically, the
function of a capacitor when it transfers the AC component of a signal from one stage
to the next, blocking the DC component.

CRT    Cathode ray tube. A picture tube, as used in older TVs and computer monitors.

current The flow of electric charges. Though it is semantically incorrect to describe
the “flow of current,” since that is the “flow of the flow of charges,” most of us in
electronics think of current as “the quantity of electricity,” and describing current as
flowing is common and considered acceptable by most practitioners. See amperes.

curse words Utterances sometimes emitted by technicians, immediately following
the appearance of magic smoke.

cutoff The state of a transistor when it is fully turned off, allowing no current to
pass through it.

cutoff frequency The signal frequency at which a transistor’s gain has decreased to 1,
or unity gain.

DAC Digital-to-analog converter, sometimes pronounced “dack.” A device that
converts digital codes to analog signals.

damper diode A diode connected across an inductor to short out reverse-polarity
current, generated when the inductor’s magnetic field collapses, which could damage
other components. Damper diodes are connected cathode to positive, so they have no
effect on the current applied to the inductor. They are most often used with relay coils
and motors. Some relays include the diode inside.

Darlington pair An arrangement of transistors with the output of one directly
feeding the base of the next, to multiply their gains. With NPN transistors, the
collectors are tied together, and the emitter of the first transistor is connected to the
base of the second. Some Darlington pairs are built into a single transistor package,
providing gain unattainable with just one normal transistor. Darlington pairs are often
found in audio power amplifiers.
288   How to Diagnose and Fix Everything Electronic

      DC-DC converter A small inverter used to step up a DC voltage to a somewhat
      higher one. Typically used to generate 15–30 volts in devices operating from 3–6 volts.

      demodulation The extraction of information from a signal. Also called detection.
      See modulation.

      digital A method of representing information by measuring it in a series of samples,
      which are then encoded into a pattern of voltages having only two states, on or off,
      representing the binary numbers 1 and 0.

      direct current (DC) The current moves only in one direction. Current moves from
      negative (an excess of electron charge) to positive (a dearth of it). Most modern gear
      uses negative ground, so the current is really moving up from the circuit ground
      point, through the components and then into the positive power supply connection.
      It’s convenient to think of ground as the sinkhole into which everything is flowing,
      though, and that’s perfectly acceptable regardless of polarity. Conceptually, the actual
      direction of electron charge flow is irrelevant. (In physical terms, the direction of flow
      is quite important, however, as you’ll disastrously discover should you try to run a
      product from a power supply with the polarity reversed.)

      discrete Made of individual parts, as opposed to integrated circuits. An audio output
      stage with separate power transistors, instead of an integrated module, is a discrete
      output stage.

      distortion The unfaithful reproduction of a signal by an amplifier, in which the
      output does not accurately mimic the input. See clipping for one example.

      DLP Digital Light Processor. The trade name of a Texas Instruments video projection
      technology employing microscopic mirrors on a chip that can be flexed with applied
      voltage, aiming reflected light toward or away from a lens. Practical DLPs may have
      millions of mirrors.

      DSP Digital signal processing. The term is applied to both the process and the specialized
      chips performing it, particularly in the filtering or enhancement of audio signals.

      DUT Device under test. The circuit or component on which you’re taking
      measurements.

      duty cycle The percentage of time that one cycle of a signal’s waveform spends in
      its “on” state. This is usually used to specify the percentage of time a square wave
      spends turned on. A wave with equal on and off times has a 50-percent duty cycle.

      emitter follower A current amplifier, or buffer, in which the output is taken from
      the transistor’s emitter. Emitter followers offer only current gain, not voltage gain, and
      they do not invert the signal. The output is a replica of the input, except that it has
      higher current driving capability, so it can feed a greater load.
                                                                       Glossary         289

envelope The overall contour of a waveform, viewed over many cycles.

ESR Equivalent series resistance. The internal resistance of a capacitor that limits
how fast it can be charged and discharged. Some ESR is normal, but electrolytic
capacitors often develop increased ESR as they age, eventually causing circuit
malfunctions.

eye pattern The distinctive envelope of the output signal from an optical disc
player’s laser head.

fall time The time it takes for a waveform to drop from 90 to 10 percent of its
maximum value.

feedback A signal sent from a circuit’s output back to its input. The path the signal
takes may involve intermediary components, and is sometimes called a feedback loop.

filter capacitor A large bypass capacitor used to smooth the output voltage of a
power supply.

frequency Specified in hertz (Hz), how many times per second an event occurs. See
alternating current.

gain The ratio of the strengths of the input and output signals of an amplifying
circuit. Voltage gain indicates that the circuit creates a replica of the signal, but with
a bigger voltage swing from peak to peak. Current gain means that the circuit’s output
signal may have the same voltage swing as its input but is able to supply more
current, as would be required, for instance, when driving a speaker.
     Also, the ratio of base-to-emitter current to the resulting collector-to-emitter current
in a transistor. For example, if 2 milliamps of base current will cause 100 milliamps of
collector current, the transistor has a gain of 50. A transistor’s gain value is engineered
into its manufacture, and it decreases with signal frequency.

giga Billions.

ground loop A condition in which the ground terminals of two connected pieces of
equipment aren’t at the same voltage level, so current passes between them, causing
AC line noise to corrupt signals. Most ground loop problems occur with analog audio
and video gear, resulting in audio hum or bars moving up the screen of a video
display. In digital systems, ground loops may result in corruption severe enough to
cause dropouts or complete loss of data transfer.

Hall-effect sensor A semiconductor transducer whose passage of current is affected
by nearby magnetic fields. These sensors are commonly used to detect rotation and
position in servo-controlled motor systems, as in VCRs and camcorders, especially
in their capstan motor assemblies. Unlike inductive sensors, Hall-effect devices will
respond to the steady field from a permanent magnet not moving relative to the sensor.
290   How to Diagnose and Fix Everything Electronic

      harmonically related Two waves that are related in frequency by a whole-number
      ratio, such as 2:1, 3:1, and so on. They are considered related because their cycles will
      have starting and stopping points that correspond in a regular, repeating pattern.

      harmonics Parts of a wave whose energies are at frequencies that are a whole-number
      multiple of the fundamental frequency of the complete wave. A square wave, for example,
      contains energy at all the odd harmonics (3rd, 5th, 7th, and so on) but none at the even
      harmonics.

      heatsink The metal structure that carries heat from an attached component,
      radiating some of the heat into the air and keeping the component cooler than it
      would be on its own. Heatsinks usually have fins to maximize surface area for greater
      cooling. Sometimes the metal chassis of the product can serve as a heatsink, with
      power-handling components bolted to it.

      impedance Usually specified in ohms, like resistance, impedance is a complex
      quantity describing the opposition to AC currents. Along with resistance, it includes
      the effects of capacitance and inductance, known as reactance. Think of it as
      resistance, but for AC. Unlike pure resistance, the impedance of a circuit varies with
      the frequency of the applied signal. The impedance of a capacitor, called its capacitive
      reactance, drops as signal frequency rises. The impedance of an inductor does just the
      opposite.

      inductance Specified in henries, inductance describes the voltage that builds up in
      a coil of wire when a changing current passes through it. The effect arises from the
      magnetic field the current creates and how that field impinges on the coil, opposing its
      passage of current. It is determined by physical factors inherent in the coil, including
      the number of turns of wire, whether it is wrapped around an iron core, and other
      conditions. In small-signal circuits, inductance is specified in millihenries (mh, or
      thousandths of a henry) or microhenries (µh, or millionths of a henry).

      input stage The low-level circuitry used to amplify small signals from transducers.

      integrated Having many components formed on the same substrate, or base layer,
      as in an integrated circuit or module.

      inversion A signal stage flips the signal upside down, so the output voltage drops as
      the input voltage rises, and vice versa. Amplifiers providing voltage gain often perform
      inversion as well, and are known as inverting amplifiers.
          This term is unrelated to the use of the word inverter to describe a step-up voltage
      converter of the sort used to drive fluorescent lamps in LCD screens.

      kilo Thousands.
                                                                    Glossary        291

land The copper area on a circuit board where components are connected, distinct
from the traces connecting such areas. Some techs call all conductive elements on
circuit boards traces.

latching or latch-up The destructive condition that may occur when an integrated
circuit’s signal input voltage exceeds the power supply voltage, causing reverse
current through the substrate.

LCD Liquid crystal display. An electronic light valve in which the molecules of
a special liquid with crystalline properties can be twisted to alter the polarization
of light passing through them, causing darkening when a fixed polarizer placed in
line no longer matches the polarization of the altered area. Practical LCDs may have
millions of individual light valve cells.

lead dress The placement and securing of wires and cables in a device.

line voltage The voltage of the AC line as it comes from the wall socket.

linear A graph of a linear circuit’s input on one axis and its output on the other
creates a straight line, indicating that the circuit is faithfully reproducing the signal,
without distortion. Especially in high-fidelity amplifiers, linearity is an important
design goal. Some circuits, such as radio mixers, are deliberately nonlinear, so that two
signals will influence each other, creating new signals.
    Also, a general term meaning analog, as opposed to digital.

load How much resistance (DC) or impedance (AC) a device presents to the circuit
driving (powering) it. The lower the resistance or impedance, the greater the load,
because more current will be needed to drive the device. Most speakers, for instance,
offer an 8-ohm load to the amplifiers driving them.

LSI Large-scale integration. A large-scale integrated circuit chip may have thousands
or millions of elements.

magic smoke The essence that actually runs electronics. Let the magic smoke out,
and the circuit won’t work anymore. All pro techs know this because they’ve made it
happen, usually to something expensive. See curse words.

matrixed Addressable in an X-Y grid, as in an LCD or a keypad.

mega Millions.

micro Millionths.

milli Thousandths.
292   How to Diagnose and Fix Everything Electronic

      modulation The altering of a signal to impose information on it. In amplitude
      modulation, or AM, a carrier’s strength changes with audio information. In
      frequency modulation, or FM, a carrier’s frequency is shifted higher and lower
      to convey the information. Many other forms of modulation are used, especially
      in digital systems. See demodulation.

      multiplexed A circuit in which information to or from separate areas is carried
      on common lines through time sequencing, or scanning. Computer keyboards and
      remote control keypads are multiplexed, with a microprocessor scanning a small set
      of lines in an X-Y style grid, looking for intersecting connections indicating which
      button has been pressed. LCDs are also scanned in such a grid. The technique permits
      a small number of lines to serve a much larger number of switches or screen pixels.
           Also, a subcarrier system used in analog FM radio to encode stereo onto a single
      carrier.

      mu-metal A special metallic alloy, particularly effective at blocking electromagnetic
      fields, used in shields over sensitive components and input stages.

      nano Billionths.

      NC No connection. Mostly used to denote IC pins that either have no connection to
      the inside of the chip or are not connected to anything in the rest of the circuit.

      negative feedback A signal fed from a circuit’s output back to its input, but upside-
      down, or 180 degrees out of phase, for controlling distortion or motion, or any other
      corrective action.

      Ohm’s Law Georg Simon Ohm’s crucial contribution to the electrical art, stating that
      voltage (E) equals current (I) times resistance (R). Knowing any two of the variables
      makes it easy to solve for the third one. Thus, I = E / R and R = E / I. Using the more
      common terms of volts, amps and resistance, V = A × R, A = V / R and R = V / A.

      open circuit or open A disabled circuit path preventing passage of current required
      for proper circuit operation, as through an open component. A switch in the “off”
      position is also said to be open, as no current passes through it.

      output stage The high-level circuitry used to deliver sufficient power to a final
      transducer or device such as a speaker, motor, transmitting antenna or backlight lamp.

      parallel Components are connected together across the power source, so current
      goes to each one, independent of the others. In a parallel circuit, the voltage is the
      same to each element, and the current that passes through each one is proportional to
      its resistance.

      passive elements The non-amplifying components supporting the operation of a
      circuit stage’s active elements. Resistors, capacitors and inductors are passive elements.
                                                                   Glossary         293

PC board or PCB Printed circuit board.

phase The relative position in time of two signals, expressed in degrees as a fraction
of one cycle’s 360-degree total. When the signals are half a cycle apart, they are
180 degrees out of phase.

photodetector A light-sensitive component, usually a phototransistor.

pico Trillionths.

pictorial A drawing of a device’s internal structure in physical terms, including
appearance and layout of its parts.

PIV Peak inverse voltage. The maximum voltage a diode or rectifier can withstand
in the direction the part does not conduct.

pixel A single picture element, or dot, especially in a matrixed display like an LCD.

polarity The direction of current, as indicated by + (positive) and – (negative).

polarized A component that requires a specific arrangement of positive and
negative voltage applied to its terminals. For example, most electrolytic capacitors are
polarized and will be destroyed if voltage is connected with the polarity backward.

power transistor A large transistor capable of passing significant amounts of
current and dissipating more than a few watts as heat. Power transistors are used as
output elements in amplifiers, as motor drivers, and anywhere else their large power-
handling capabilities are required.

push-pull A common form of high-fidelity audio amplifier in which the positive and
negative halves of the signal waveform are amplified separately and recombined at
the output. See complementary.

PWM Pulse-width modulation. The technique of varying the duty cycle, or on/
off ratio, of a series of pulses to convey information or control the flow of power.
Switching power supplies use PWM to regulate their output voltages, and PWM is also
used to vary the speed of motors in some applications.

quiescent current The current drawn by an amplifier stage when no signal is
present. Quiescent current is set by the active element’s bias.

radio-frequency (RF) Generally from around 100 kHz to many GHz (gigahertz, or
billions of cycles per second).
294   How to Diagnose and Fix Everything Electronic

      rail The power supply line feeding a circuit. Though some circuits with inductors
      can generate voltages exceeding those of the rails powering them, most cannot, and
      the rail’s voltage is considered the circuit’s limit.

      rectify To convert AC to DC, either by chopping off one half of the waveform or
      directing opposite halves to the appropriate + and – terminals, so that neither output
      terminal changes polarity as the incoming current alternates.

      resistance Specified in ohms, resistance opposes the current moving through any
      conductor or circuit, limiting the total amount. It is essentially atomic friction, and
      the power lost to it is converted to heat. It is represented as R and by the Greek letter
      omega, Ω. Resistance has no polarity, just as a crimp in a hose has the same effect
      regardless of the direction of water flow.

      resonance The state in which a component or combination of components builds
      up a voltage or current when an AC signal of a specific frequency is applied, because
      the transit time of the energy through the circuit is such that energy peaks reflecting
      back and forth through the components coincide with the incoming peaks of the
      applied signals, reinforcing them. The electrical effect is similar to the mechanical
      effect that occurs in a musical instrument’s string, and resonant circuits are said to be
      tuned to their resonant frequency.

      ripple The unwanted variations in the output of a DC power supply.

      rise time The time it takes for a waveform to rise from 10 to 90 percent of its
      maximum value.

      RoHS Restriction of Hazardous Substances. The European standard specifying lead-
      free soldering and other environmentally safer construction characteristics. Devices
      built to this standard will display RoHS on a label or stamped into the case.

      sampling The process of taking rapid measurements of analog voltages to convert
      them to digital data representation. See digital.

      saturation The state of a transistor when it is fully turned on, allowing maximum
      current to pass through it with minimum resistance.

      sawtooth wave An asymmetrical waveform whose shape resembles the teeth on a
      saw, typically used in the beam-sweeping circuitry of CRT TV sets and oscilloscopes.

      schematic A circuit diagram showing the symbols and interconnections of
      components, without regard to their size, appearance or physical layout in the device.

      selectivity The ability of a receiver to separate adjacent stations.
                                                                    Glossary         295

semiconductor A component, usually made from silicon, such as a transistor, diode
or IC chip. Some exotic semiconductors are made from other materials like gallium
arsenide or germanium, which behave similarly to silicon in their ability to control
the flow of electrons in response to a control signal.

series Components are connected end-to-end, so current must pass through one
to get to the other. A fuse, for instance, is connected in series with the circuitry it’s
protecting, so that all current must pass through the fuse to reach the rest of the
components.
    In a series circuit, the current is the same through each element, but the voltage
reaching each element is reduced in proportion to the other elements’ resistance. The
lowering of voltage is referred to as an element’s voltage drop.

series pass transistor A transistor used as a variable resistance element in a linear
voltage regulator to hold the output voltage to a specific value, with all of the power
supply’s current passing through the transistor on its way to the rest of the circuitry.

shield A metal cover placed over sensitive circuitry to protect it from stray electrical
noise. Shields are often soldered in place and must be removed for access.

short circuit or short A low-resistance passage of current where it shouldn’t exist,
as from a voltage point directly to circuit ground via a shorted component.

shunt A current path in parallel with another path. A resistor across another
component, diverting some current around it, is said to be shunting the component or
shunting the current.

signal A changing voltage representing information.

sine wave A waveform with a smooth, repetitive shape derived from the mathematical
sine function. Sine waves have no harmonics, or energy at multiples of their frequency,
so they are the purest type of signal possible.

sink An acceptor of current that permits it to pass toward ground. See source.

sled The linear track assembly on which a laser optical head rides as it scans across
the disc. The sled includes the track rods and the motor and gears moving the head.

SMD or SMT Surface-mount device or surface-mount technology. These are the tiny,
leadless components that have replaced conventional through-hole parts with leads.

SMPS Switch-mode power supply. Another name for a switching power supply.

SNR or S/N Signal-to-noise ratio. The ratio of desired signal to residual noise in
a circuit, usually expressed in decibels (db).
296   How to Diagnose and Fix Everything Electronic

      source A supplier of current. See sink.

      spectrum A range of frequencies.

      square wave A waveform with flat tops and bottoms that switches quickly between
      the two states, with fast rise and fall times. Square waves can never be truly square,
      because it takes some time for them to switch states.

      stage The distinct section of a circuit that performs one function of signal processing.
      Circuit stages are organized around one or more active elements, with surrounding
      passive elements supporting their operation.

      subcarrier A carrier signal imposed on another, higher-frequency carrier. The
      technique is used to piggyback hidden signals onto others, as in FM stereo and analog
      color video signals.

      substrate The base on which an integrated circuit, transistor or potentiometer is
      formed. In most modern solid-state components, the substrate is a slice of silicon.

      switch mode The operation of a transistor as a switch—that is, saturated or cut
      off—with fast rise and fall times between the two states.

      sync The pulses in an analog video signal that align the position of a TV’s display
      point (the electron beam’s position in a CRT TV or the row and column addresses in a
      matrixed display like an LCD or plasma TV) with that of the originating signal source,
      to keep picture elements correctly placed.

      time constant The period of time it takes to charge or discharge a combination of
      components, such as a resistor and capacitor or an inductor and capacitor.

      tough dog A unit with a repair problem that’s difficult to diagnose.

      trace The conductive copper lines on a circuit board providing connection between
      lands, or component connection points.
          Also, to follow a signal through a circuit with a measuring instrument, especially
      an oscilloscope, or to follow a connection visually across a circuit board or a schematic
      diagram.

      transducer A device that takes in one kind of information or energy and converts
      it to another. Microphones, tape heads, laser optical heads, speakers and phono
      cartridges are all transducers.

      video An analog signal representing a moving image by varying a voltage to represent
      the brightness and color values of scanned spots in the image. Video signals are complex
      and include synchronizing and color reference signals to align the receiver’s circuits as
      necessary to interpret the signal properly and display the image.
                                                                        Glossary        297

   volt-amps (VA) This is somewhat like watts, but for AC power. It means “volts times
   amps.” Because of certain effects that occur with AC, the maximum voltage peak and
   maximum current in each cycle may not coincide, and the amount of work that can be
   done will be less than when they do. The power requirements shown on the backs of
   AC-powered devices are often specified in VA. For repair work, it’s safe to think of VA
   and watts as equivalent, since actual watts used will never exceed VA and may be less.

   volts The electric “pressure” or electromotive force. This corresponds to how hard
   the electrons push, and is represented as V or sometimes E. Voltage propels current
   through resistance, so the higher the voltage, the more current will pass through a
   given resistance. (See Ohm’s Law). Vcc and Vdd are references to voltages applied
   to the pins of a semiconductor, and mean the same thing as V, except that they are
   always of positive polarity. Negative voltage is sometimes noted as Vss.
        Like all other motion of mass in the universe, voltage is entirely relative; there is
   no such thing as “9 volts.” A given voltage is meaningful only relative to some other
   point. Polarity is relative too; a voltage can be positive with respect to one point while
   being negative with respect to another! In circuits that have both positive and negative
   voltages relative to circuit ground, that ground point will function as positive for the
   negative voltage point and negative for the positive voltage point.

   watts Also called power, a measure of how much work can be done by the electricity.
   Power is equal to volts × amps. So, 100 watts could be 20 volts at 5 amps or 10 volts at
   10 amps; the amount of work either of those quantities could do would be the same.
   746 watts equal one horsepower, but if you cause 746 watts of electrical power to flow
   through an actual horse, you wind up with a dead horse and zero horsepower.

   Watt’s Law This states that power in watts (P) equals current (I) times voltage (E).
   As with Ohm’s Law, knowing any two of the variables makes it easy to solve for the
   third one. Thus, I = P / E and E = P / I. Or, in the terms of watts, amps and volts,
   W = A × V, A = W / V and V = W / A.



Common Circuits
   Many devices are constructed from the same basic types of building blocks. Here are
   some widely used circuits you may find.

   amplifier One or more stages that increase a signal’s voltage swing, current or both,
   by shaping power supply current into a larger replica of the signal.

   buffer An intermediary circuit isolating two others. In analog devices, an amplifier
   stage providing current gain and isolating the more sensitive, earlier amplifier stage
   from the ones following it. Buffers typically offer no voltage gain. In digital processing,
   a temporary storage area holding blocks of data, perhaps in line for conversion to
   analog signals.
298   How to Diagnose and Fix Everything Electronic

      clock oscillator The square wave oscillator used to step, or clock, a digital circuit
      through its series of operations. Many microprocessors used in small products include
      onboard clock oscillators, requiring only a crystal or ceramic resonator to set the
      frequency.

      crowbar A circuit designed to detect a fault condition, usually overcurrent or
      excessive voltage, and deliberately blow the power supply’s fuse to stop operation.

      frequency synthesizer A complex circuit used to generate a range of frequencies
      from the single frequency of a quartz crystal. Older frequency synthesizers used
      voltage-controlled oscillators and phase-locked loops to lock an analog oscillators’ signals
      to the frequency determined by a digital controller. Many newer systems use direct
      digital synthesis, in which the required analog waveforms are generated by the
      digital system via a digital-to-analog converter in much the same way a CD player
      reconstructs the analog audio waveform from digital samples.

      front end The sensitive first stages of a radio receiver that accept energy from the
      antenna and amplify it in preparation for its delivery to the mixer. Many front ends
      include tuned circuits to help reject off-frequency signals, but those in frequency-
      synthesized receivers may not.

      intermediate-frequency (IF) The fixed-frequency, tuned amplifier stages in a radio
      receiver providing most of the selectivity (ability to separate stations) and sensitivity.
      Incoming signals from the antenna are converted to the IF by mixing them with a local
      oscillator in the mixer stage, a deliberately nonlinear circuit that causes the two signals
      to interfere, generating a new signal at their difference frequency that still carries the
      information of the original signal.
           The mixing process is also called heterodyning, and radios that convert signals to
      an IF, which virtually all modern sets do, are called superheterodyne receivers, because
      the output of the heterodyne process is still higher up the frequency spectrum than
      the information carried by the signal.
           Also, the frequency at which the tuned amplifiers operate.

      inverter A step-up switching power supply typically used to generate the high
      voltages required for fluorescent LCD backlights, camera flash tubes and other
      applications needing voltages higher than those provided by the product’s primary
      power supply or batteries.

      linear voltage regulator A step-down regulator (one whose output is at lower
      voltage than its input) using a transistor element in linear mode, as a variable resistor
      that dissipates excess power as heat.

      mixer A circuit that accepts two AC signal inputs and outputs the sum and difference
      frequencies resulting from their interacting, or mixing, with each other in a nonlinear
      manner.
                                                                       Glossary        299

oscillator A circuit that produces a steady signal of constant frequency, or variable
frequency in some cases, through the use of constructive, or reinforcing ("positive"),
feedback.

phase-locked loop (PLL) A multistage circuit used to slave the frequency of an
oscillator to that of a reference signal or another oscillator. By inserting digital dividers
between the oscillator and the rest of the loop, the oscillator can be forced to lock to a
multiple of the reference frequency, a crucial technique in frequency synthesis. PLLs
are used to tune receivers, to recover color information from analog videotape and data
from the laser head’s signals in CD and DVD players, and in many other applications.

power supply Any circuit or power source that provides the power the rest of the
device requires. In AC-powered products, it typically converts the incoming line
voltage to a steady, lower DC voltage. In battery-operated products, the batteries
themselves may be considered the power supply. If intermediate circuitry steps the
battery voltage up or down, that circuitry will also be considered part of the device’s
power supply.

preamplifier or preamp A low-level amplifier used to boost weak signals enough
that further stages can amplify them more.

servo A circuit that slaves mechanical motion to some reference, typically used to
lock the rotational speed or phase of a motor to a signal, as with a VCR’s rotating video
head drum, which must be kept aligned to the recorded tape tracks for playback.

switching or switch mode power supply A power supply operating in switch mode,
in which the incoming power is chopped into high-frequency pulses before being fed
to the conversion transformer. This approach allows a small transformer to provide
substantial power output, and it is very efficient when compared to a linear power
supply, which processes power at the very low line frequency of 50 or 60 hertz.
Switching power supplies can be used to step voltage up or down. Step-up versions are
often referred to as inverters.

switching or switch mode voltage regulator A voltage regulator operating in
switch mode, in which the duty cycle of a pulse is varied to regulate the output voltage.
This is similar to the operation of a switching power supply, except that the incoming
power is already at a DC voltage in the neighborhood of the desired output voltage.
Compared to a linear voltage regulator, a switching regulator generates far less heat
and is much more efficient. Plus, it can be used to step the voltage up or down.

voltage-controlled amplifier (VCA) An amplifier whose gain can be controlled
by a DC voltage. VCAs are used in the audio sections of digital devices such as MP3
players, with the DC voltage from the microprocessor setting the headphone volume.

voltage-controlled oscillator (VCO) An oscillator whose frequency can be
controlled by a DC voltage. VCOs are used in radio tuning circuits, with the DC
voltage from the digital circuits setting the frequency.
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           Index
A                                         ADC (analog-to-digital converter),
                                             65, 285
                                                                                     amplitude modulation. See AM
                                                                                        (amplitude modulation)
a/c (audio/control) head, VCRs and        add mode, Vertical Mode oscilloscope       analog
   camcorders, 271, 275–276                  control, 111                                  defined, 285
AA batteries, 103, 105, 110               AF (audio-frequency), 285                        power amplification in audio
AAA batteries, 48, 105                    alcohol, setting up workplace, 20                   amplifiers, 236
AC adapters                               alkaline batteries                               servicing with signal generator, 23
      defined, 48                               cleaning up leakage, 49                    signal processing, 193
      diagnosing dead units, 200                digital cameras/MP3 players not      analog ground symbol, schematics, 151
      measuring current with                       operating with, 105               analog meters
         DMMs, 75                               overvoltage from, 47                       inaccuracy of, 105
      measuring resistance with           aliasing in digital oscilloscopes, 10–11         using, 109–110
         DMMs, 73–74                      All Electronics, 223                             workplace setup, 24–25
AC (alternating current)                  Alt or Alternate control,                  analog oscilloscopes
      checking cord safety after             oscilloscope, 88                              with cursor measurement, 12
         repairs, 38                      Alt trigger control, oscilloscope, 89            digital vs., 10–11, 96–98
      as component of DC voltage, 85      alternating current. See AC                      overview of, 9–10
      defined, 61, 285                       (alternating current)                         with storage, 12
      electric shock from products        AM (amplitude modulation)                  anatomical analogies
         with, 198                              defined, 292                               attention to detail in repair
      finding power supply in                   example of silent shortwave, 56               jobs, 42
         products with, 189–191                 reading radio schematics, 164              brains of modern products, 41
      impedance in. See impedance         amperes (amps), 60, 285                          diagnosing machine by taking
      line voltage, 291                   amplifier stage, reading schematics,                history, 51–52
      measuring voltage with                 154–158                                       machines as systems, 39–40
         DMMs, 73                         amplifiers                                 antennas
      oscilloscope input coupling               as common circuit, 297                     disassembly of cell phones, 181
         setting, 84–88                         common types of, 65–68                     fixing laptop Wi-Fi, 260
      oscilloscope trigger control, 90          disassembly of, 177                        schematic symbol for, 150
      reactance and, 61                         protecting hearing during repair     arc lamps, video projectors, 277–278
      schematic symbol for voltage, 150            of, 34                            Astigmatism or astig control,
accuracy                                        recognizing output stage,               oscilloscope, 78
      digital multimeter, 8, 71–72                 194–195                           Astigmatism or astig, oscilloscope
      digital vs. analog scopes, 97             of vacuum tube volt meters, 23          screen control, 80
      frequency counter, 108              amplitude                                  audio
      oscilloscope not intended for, 77         in AC coupling, 86–87                      defined, 285
active elements, circuit stages                 defined, 285                               diagnosing alive and awake but
      amplifier stage, 155                      measuring with cursor                         not quite kicking units, 203
      defined, 63, 285                             controls, 96                            fixing in VCRs and camcorders,
      diagnosing cause and effect, 208          in signal tracing and                         276–277
      diagnosing nonfunctional units               diagnosis, 207                    audio amplifiers
         by stages, 206–207                     signals varying in, 63                     failure from heat stress, 47
                                                                                           malfunctions in, 162–163


                                                                                                                      301
302           Index
audio amplifiers (cont.)                   Bandwidth Limit vertical control,              markings, 121
      recognizing output stage features,      oscilloscope, 87                            stocking up on, 28
          194–195                          base element, bipolar transistors, 140         in switching supply failure, 160
      schematics, 161–162                  baseband video, radio, 191–192           Brightness or Intensity, oscilloscopes,
      using bipolar transistors for        basements, as workplaces, 6                 78–80
          output stages, 142               batteries                                buffers
audio amplifiers and receivers, tips and         backup, diagnosing dead                  as common circuit, 297
  tricks                                            units, 201                            connecting frequency counter to
      dangers within, 238                        digital multimeter, 72                       circuit, 109
      how they work, 236–237                     disassembly of cell phones, 181          op amps as, 128
      how to fix, 238–240                        disassembly of laptops, 184        bulbs
      if it is worth fixing, 238                 disassembly of MP3 players, 181          desoldering with rubber
      what can go wrong, 237                     disassembly of PDAs, 181                     solder, 17
audio/control (a/c) head, VCRs and               disassembly, removal of, 172             as solder suckers, 102
  camcorders, 271, 275–276                       electric shock from, 198           bus, defined, 286
audio-frequency (AF), 285                        finding voltage converters/        bypass capacitors
audio taper pots, resistance on, 134                regulation systems in, 188            defined, 62–63, 286
auto setup, digital scopes, 97                   laptop failure and, 254                  in digital signal processing, 192
Auto sweep mode control,                         laptop failure, fixing charging          filter capacitors as large, 289
  oscilloscope, 91                                  problems, 256–257                     in signal tracing and
autoranging, DMMs, 72–74                         laptop failure, when it is worth             diagnosis, 209
                                                    fixing, 255
B                                                leakage damage from, 49            C
                                                 MP3 player problems, 261
B Ends A, Delayed Sweep control,                                                    cables
                                                 MP3 player problems, fixing, 262
   95–96                                                                                  laptop failure and, 255
                                                 power supplies, recognizing
backlight inverters                                                                       laptop failure, display problems,
                                                    components, 187–188
      avoiding electric shock, 33                                                             258–259
                                                 power supplies, voltage
      disassembling DVD players with                                                      laptop failure, fixing, 260
                                                    settings, 105
         LCD screens, 180                                                                 laptop failure, when it is worth
                                                 power supply connections,
      disassembling laptops, 184                                                              fixing, 255
                                                    103–105
      failure in switchers, 234                                                           MP3 player problems, 261
                                                 product failure from
      finding parts, 223                                                                  reassembly of, 228–229
                                                    overvoltage, 47
      fixing flat-panels, 250–251                                                   call numbers, 152–153
                                                 product failure from reversed
      heat stress in, 47                                                            camcorders
                                                    polarity, 48
      laptop failure and, 254                                                             audio problems, 275–276
                                                 schematic symbol, 150
      laptop failure, dangers of                                                          cleaning, 269–270
                                                 terms used for, 60
         fixing, 256                                                                      color problems, 275
                                           Beam Finder, oscilloscopes, 80–81
      laptop failure, display problems,                                                   dangers within, 266
                                           belts
         258–259                                                                          digital recorders, 274
                                                 fixing doors, disc players and
      laptop failure, when it is worth                                                    disassembly of, 181–182
                                                    recorders, 243
         fixing, 255                                                                      how they work, 263–264
                                                 turntable, 178
      LCD, 191                                                                            if it is worth fixing, 265
                                           bench vise, workplace setup, 26
      output connection dangers, 198                                                      other loading problems,
                                           BGAs (ball grid arrays)
      replacing SMT output transistors                                                        268–269
                                                 laptop failure, display
         in, 218                                                                          recognizing components, 196
                                                    problems, 259
backup batteries, 201                                                                     servo problems, 274–275
                                                 laptop failure, fixing, 256
ball grid arrays. See BGAs (ball grid                                                     stuck tapes, 266–268
                                                 laptop failure from, 253
   arrays)                                                                                tracking problems, 270–275
                                           bias
ballasts                                                                                  what can go wrong, 265
                                                 in amplifiers, 66
      bench-testing repaired                                                        capacitance, 61, 202, 286
                                                 defined, 286
         projector, 232                                                             capacitance meters, 23, 106–107
                                                 with diodes and rectifiers, 121
      CCFTs not requiring, 234                                                      capacitive reactance, 61, 290
                                           bipolar circuits, 286
      defined, 285                                                                  capacitors
                                           bipolar transistors, 140, 142, 218–219
      fixing video projectors, 280–281                                                    adjusting to cover real trouble,
                                           blobs, in video projector image, 283
bandwidth                                                                                     42–43
                                           block diagrams, 147–148, 163
      choosing digital oscilloscope, 12                                                   avoiding electric shock,
                                           bridge rectifiers
      oscilloscope measuring, 9, 87                                                           32–33
                                                 defined, 120
                                                                                                        Index            303
      call number, 152                          failure from dead, 160, 235                   signal generators checking, 107
      checking with capacitance meter,          recognizing, 189                              signal processing in digital
          23, 106–107                     circuit boards. See also PCBs (printed                 devices, 194
      defined, 61, 113                       circuit boards)                                  simulating in digital devices, 23
      diagnosing cause and effect, 207          disassembling MP3 players,             clocking
      electrical damage from, 35–36                180–181                                    checking comatose or crazy units
      fixing laptop computers, 256              disassembling PDAs, 181                          for, 202
      markings on, 113–115                      fixing hard drives, 252                       checking dead units for, 200–201
      out-of-circuit testing on, 117            high voltages of TV and LCD                   crystal clock oscillators as
      product failure from, 49–51                  monitors, 178                                 complete circuits for, 118
      stocking up on, 27–28                     intermittents caused by cracks in,            oscillators providing, 68
      substituting for, 215–216                    204–205                             CLV (constant linear velocity), CDs and
      uses for, 115                             physical stress to, 49                    DVDs, 165
      what kills them, 115–117                  power supply, 189–190                  CMOS (complementary metal oxide
capstan servo, 264                              reconnecting, 228                         semiconductor), 286–287
carbon film type resistors, 131–132       circuit boards, replacing components         coaxial power plugs, 103–104
carpeting, workplace setup, 6                   from dead machines, 214                coils, reading inductance value of, 29
carrier signals                                 finding parts, 223                     cold-cathode fluorescent tubes
      affect of modulation on, 292              installing new parts, 220–222             (CCFTs), 286
      defined, 286                              LSI chips, 224–225                     cold solder joint
      multiplexed circuits and, 292             overview of, 211                              defined, 98, 100
      in subcarriers, 296                       saving damaged, 223–224                       vibration or position-sensitive
cathode ray tubes. See CRTs (cathode            from stockpile, 214                              intermittents caused by, 204
   ray tubes)                                   substituting with close match,         collector element, bipolar
CCFTs (cold-cathode fluorescent                    215–220                                transistors, 140
   tubes), 286                                  surface-mount components,              color-coded markings, resistors,
CD players and recorders                           213–214                                131–132
      dangers within, 242–243                   through-hole parts, 211–213            color problems, VCRs and
      disassembly of, 177                 circuit ground points                           camcorders, 275
      how they work, 240–241                    avoiding electric shock,               color wheels
      how to fix, 243–248                          31–32                                      DLP TV/video projectors, 195, 197
      if it is worth fixing, 242                checking DC voltage with DMMs,                fixing DLP projectors, 276–277,
      protecting vision when                       72–73                                         282–283
          repairing, 33                   circuits                                     columns, on flat-panel screens, 249
      reading schematics, 164–165               adjusting to cover the real trouble,   comatose units, diagnosing, 202
      what can go wrong, 241–242                   42–43                               complementary
cell phones, disassembly of, 181                amps moving through, 60                       amplifiers, 66
ceramic disc capacitors                         building blocks, 65–68                        defined, 287
      almost never fail, 115                    concepts, 61–63                        complementary metal oxide
      defined, 115                              connecting frequency counter              semiconductor (CMOS), 286–287
      markings on, 114                             to, 109                             complete (closed) circuits, 62
      not necessary to have spare, 28           getting inside to. See disassembly     component cooler spray
Ch 1 mode, oscilloscope                         types of, 297–299                             in thermal intermittent units,
      trigger control, 89                 clamp problems, disc players and                       203–204
      vertical position, 78                  recorders, 244                                   using, 111
      volts/div or attenuator knob, 78    Class A amplifiers, 66                              workplace setup, 27
Ch 2 trigger control, oscilloscope, 89    Class AB amplifiers, 66                      component video, TV, 192
charging problems, laptop computers,      Class D digital amplifiers, 236              components
   256–257                                cleaning, VCRs and camcorders,                      capacitors, 113–117
chassis ground symbol, schematics, 151       269–270                                          crystal clock oscillators, 118–120
chemicals, avoiding inhalation of, 34     clip leads, workplace setup, 19–20                  crystals and resonators, 117–118
child-safe workplaces, 5                  clipping                                            diodes, 120–122
Chip Quik, 18                                   amplifying analog signals                     fuses, 122–124
chips, diagnosing nonfunctional                    and, 66                                    inductors and transformers,
   stages, 206                                  defined, 286                                     124–126
Chop vertical control, oscilloscope, 88   clock oscillators                                   integrated circuits, 126–128
choppers, switching power supplies              crystal, 118–120                              op amps, 128–130
      defined, 159, 286                         defined, 298                                  potentiometers, 133–135
304           Index
components (cont.)                         CRT TVs                                      DC (direct current)
      recognizing. See features,                 avoiding electric shock, 32–33               defined, 60, 288
         recognizing major                       disassembly of, 177                          measuring resistance with
      relays, 135–137                            using sawtooth waves, 64–65                     DMMs, 74
      removing/replacing on circuit        CRTs (cathode ray tubes)                           measuring voltage with DMMs,
         boards. See circuit boards,             analog oscilloscopes, 9, 12                     72–73
         replacing components                    avoiding electric shock, 32–33               measuring with DMMs, 75–76
      resistors, 130–133                         defined, 287                                 oscilloscope input coupling
      schematics. See schematics                 early digital oscilloscopes with, 10            setting, 84
      switches, 137–139                          oscilloscope screen controls,                oscilloscope trigger control, 90
      transistors, 139–142                          79–81                                     schematic symbol for
      voltage regulators, 143–144          crystal clock oscillators, 118–120                    voltage, 151
      zener diodes, 145–146                crystals                                     DC offset, audio amplifiers and
composite baseband video, TV,                    call numbers, 152                         receivers, 237, 239
   191–192                                       overview of, 117–118                   DDS (direct digital synthesis), 68, 298
conductive glue, 46                        cups, small plastic                          dead units, diagnosing, 199–201
conductors, schematic symbols for,               disassembly, laptops, 183–185          deflection, and oscilloscopes, 77
   150–151                                       disassembly, PDAs, 181                 delay time multiplier, Delayed Sweep
connections                                      disassembly, removing screws              control, 95
      fixing in audio receivers, 240                layer by layer, 175–177             delay time or B timebase, Delayed
      how flat-panels can go                     disassembly, tape-based                   Sweep, 93
         wrong, 249                                 camcorders, 181–182                 delayed sweep, oscilloscopes
      power supply, 103–105                      reassembly using layers and, 231             analog, 9–10
      product failure from bad, 45               repairing broken latches with,               B Ends A, 95–96
      vibration or position-sensitive               230–231                                   delay time multiplier, 95
         intermittents causing bad,              workspace setup, 21                          delay time or B timebase, 93
         204–205                           current (amperage)                                 digital, 10
connectors. See also ribbon cables and           amount moving through circuit, 60            horizontal display mode, 93–95
   connectors                                    defined. See also amperes (amps)             overview of, 92–93
      call numbers, 152                          measuring with DMMs, 8, 75–76          demodulation, 288
      fixing hard drives, 252–253                power supply, 22, 105                  DeoxIT contact cleaner spray, 20
      physical damage to, 36                     substituting/replacing diodes or       depletion mode, FET transistors, 140
      pulling wire, 175                             rectifiers, 216–217                 design errors
      reassembly errors, 228                     workplace setup, 7                           causing electrical stress, 48
constant linear velocity, 241              current blasting, when all else fails,             infant mortality in products from,
consumer electronics, 149                     209–210                                            44–45
contact cleaner spray, 20, 110             current gain                                 desoldering tools
contamination, removing soldering                defined, 289                                 desoldering braid, 16–17
   iron, 99–100                                  emitter followers offering only, 288         removing surface-mount
continuity, measuring with DMMs,                 push-pull audio amplifiers, 162                 components on boards,
   74–75                                   cursor controls, oscilloscopes, 12, 96                213–214
control track, 264                         cut off state, 67, 287                             removing through-hole parts,
cooler spray. See component cooler         cutoff frequency, 220, 287                            211–213
   spray                                   cutters (dikes), 19                                rework stations, 102
cotton swabs, 20                           cyanoacrylate glue, workplace setup,               suckers, 102
coupling                                      26–27                                           wick, 101–102
      in audio amplifiers and receivers,   cycles per second, 61                              workplace setup, 16–18
         236, 239                          cycles, waveform, 63                         device under test (DUT), 22, 288
      defined, 63, 287                                                                  diagrams. See schematics
Coupling trigger setting,                  D                                            diameter, solder, 16
   oscilloscopes, 79, 89                                                                Digikey, 223
                                           DACs (digital-to-analog converters), 287
crashes, fixing laptop computers, 256                                                   digital cameras
                                           damper diodes, 287
crazy units, diagnosing, 202                                                                  disassembly of, 182–184
                                           Darlington pairs, 287
crossover distortion, push-pull audio                                                         electric shock from, 198
                                           data books, workplace setup, 27
   amplifiers, 162                                                                            finding voltage converters and
                                           DC-DC inverter, 288
crowbar circuits, 161, 298                                                                       regulation systems in, 188
                                                                                                           Index           305
      physical stress damaging, 49–50       disc players and recorders                     DUT (device under test), 22, 288
      taking photos before                        CD players. See CD players and           duty cycle
          disassembling devices, 22                   recorders                                 defined, 288
digital, defined, 288                             DVD players. See DVD players                  in signals, 64
digital frequency synthesizer, 67–68                  and recorders                             switching power supply, 159
digital gates, integrated circuits, 67      discrete (nonintegrated-circuit) stages        DVD players and recorders
digital ground symbol, schematics, 151            defined, 288                                  dangers within, 242–243
Digital Light Processor. See DLP (Digital         transistors in, 142                           disassembly of, 177, 179–180
   Light Processor)                               transistors in output stage, 194              how they work, 240–241
digital multimeters. See DMMs (digital      discs, hard drive failure, 251                      how to fix, 243–248
   multimeters)                             display, fixing laptop, 258–259                     if it is worth fixing, 242
digital oscilloscopes, 10–12, 96–98         distortion                                          protecting vision when
digital recorders, fixing, 274                    clipping as type of, 67, 286                      repairing, 33
digital sampling (digitizing),                    defined, 288                                  reading schematics, 164–165
   oscilloscopes, 10                        divs (division), calibrating                        what can go wrong, 241–242
digital signal processing. See DSP             oscilloscope, 77                            dynamic transistor testers, workplace
   (digital signal processing)              DLP (Digital Light Processor)                    setup, 23
digital signals, 65                               color wheel assembly in, 195
digital-to-analog converters (DACs), 287          color wheel assembly in TVs, 197         E
dikes (cutters), 19                               defined, 288
                                                                                           earth ground symbol, schematics, 151
diodes                                            overview of, 276
                                                                                           eBay, finding parts on, 13–14, 25
      call number for, 152                  DLP projectors
                                                                                           ECB (emitter, collector and base),
      choosing substitutes, 216–217               how to repair. See video
                                                                                              substituting transistors, 218–219
      diagnosing nonfunctional units by               projectors, tips and tricks
                                                                                           eco bulbs, workplace setup, 7
          stages, 206–207                         removing case, 179
                                                                                           ecological reasons, repairing for, 3
      LEDs, 121                             DMMs (digital multimeters)
                                                                                           ECOs (engineering change orders),
      overview of, 120–122                        batteries, 72
                                                                                              48–49
      stocking up on, 28                          built-in capacitance
                                                                                           electric shock
      switching supply failures and, 160              measurement, 23
                                                                                                 avoiding, 31–33
      testing with DMMs, 76                       checking supply’s output voltages
                                                                                                 from digital camera flash
      zener. See zener diodes                         with, 200
                                                                                                    capacitors, 183–184
direct-coupled circuit, 162                       diode test with, 76
                                                                                                 in major components, 198
direct current. See DC (direct current)           measuring AC voltage, 73
                                                                                                 from oscilloscopes, 77
direct digital synthesis (DDS), 68, 298           measuring continuity, 74–75
                                                                                                 from TV and LCD monitors, 178
disassembly                                       measuring DC current, 75–76
                                                                                                 from video projectors, 279
      camcorders, 181–182                         measuring DC voltage, 72–73
                                                                                           electrical damage to equipment, 34–36
      cell phones, 181                            measuring resistance, 73–74
                                                                                           electrical energy, terms, 59–61
      digital cameras, 182–183                    measuring things that don’t
                                                                                           electrical power, workplace setup, 6–7
      laptop computers, 183–185                       change quickly, 71
                                                                                           electrical pressure, 60
      layers and photos, 175–177                  overview of, 8
                                                                                           electrical stress, 47–49
      MP3 players, 180–181, 262                   setting power supply voltage
                                                                                           electrical tape, 21, 38
      overview of, 171–172                            with, 105
                                                                                           electrolytic capacitors
      PDAs, 181                                   transistor testers on some, 106
                                                                                                 defined, 115
      portable DVD players with LCD         door problems, disc players and
                                                                                                 diagnosing alive and awake but
          screens, 179–180                     recorders, 243, 246
                                                                                                    not quite kicking units, 203
      pulling wire connectors, 175          double diodes, 120
                                                                                                 diagnosing comatose or crazy
      receivers and amplifiers, 177         double rectifiers, 120
                                                                                                    units, 202
      recognizing features. See features,   DPDT (double-pole, double-throw)
                                                                                                 diagnosing dead units, 200
          recognizing major                    switch, 138
                                                                                                 diagnosing nonfunctional
      removing ribbons, 174–175             driver transistors, 162
                                                                                                    stages, 206
      rules, 172–173                        dropout compensator, 71–72, 263
                                                                                                 discharging to avoid electric
      separating snaps, 173–174             DSP (digital signal processing)
                                                                                                    shock, 32–33
      turntables, 178                             audio amplifiers and
                                                                                                 failure from overheating, 47
      TVs and LCD monitors, 177–178                   receivers, 236
                                                                                                 failure of, 115–116
      VCRs, CD and DVD players, 177               defined, 288
                                                                                                 finding in battery-powered
      video projectors, 178–179                   overview of, 192–194
                                                                                                    gear, 188
disc motors, CD and DVD player                    reading radio schematics, 164
                                                                                                 fixing flat-panels, 250
   schematics, 165                          dust filters, video projector, 278, 280, 281
306           Index
electrolytic capacitors (cont.)                    comatose or crazy, 202             focus, laser problems in DVD
      fixing switchers with bad, 235               dead, 199–201                         players, 246
      out-of-circuit testing on, 117               desperate measures, 208–210        Focus, oscilloscope screen control, 78, 80
      recognizing power supply, 189                diagnosing by stages, 206–207      forward voltage, substituting/replacing
      stocking up on, 28                           diagnosing cause and effect,          diodes, 216
      substituting for, 215–216                        207–208                        fractures inside components, 204
      thermal, 204                                 intermittent, 203–205              Franklin, Benjamin, 59
electromotive force. See also voltage              working backward or forward,       frequency
   (volts), 60                                         205–206                               crystals and resonators marked
electrons                                   fall time, waveforms                                with, 118
      avoiding electric shock, 31–32               defined, 289                              defined, 61, 289
      moving electrical current and, 60            square waves, 64, 296                     setting oscillator, 67
      when machines work properly, 40              switch mode and, 296                      spectrum analyzer voltage vs., 30
emitter, collector and base (ECB),          families, small-scale integrated                 waveform, 63
   substituting transistors, 218–219            circuit, 128                          frequency counters, 24, 107–109
emitter element, bipolar                    fans, video projector                     frequency modulation. See FM
   transistors, 140                                fixing overheating problems,          (frequency modulation)
emitter followers, 66, 288                             281–282                        frequency reference, frequency
enhancement mode, FET                              what can go wrong, 277–278            counter, 108
   transistors, 140                         farads                                    frequency synthesizer
envelopes, 11–12, 289                              markings on capacitors, 113–114           as common circuit, 298
equivalent series resistance. See ESR              reading capacitance meters, 107           radio schematics, 164
   (equivalent series resistance)           features, recognizing major                      signal processing in radios, 193
equivalent-time sampling, 12                       danger points, 198                 friction, as resistance, 60
errors                                             input sections, 191–192            front ends
      made by beginners, 42–44                     mechanisms, 195–197                       amplifiers, 67
      reassembly, 227–228                          output stages, 194–195                    defined, 298
ESR (equivalent series resistance)                 overview of, 187                          reading radio schematics, 164
      capacitance meters reading, 107              power supplies, 187–191            function generators, 23
      checking electrolytic capacitors in          signal processing, 192–194         fuses
         dead diagnosis, 200                feedback, or feedback loop, 289                  call numbers, 152
      defined, 289                          ferrite materials, battery-powered               diagnosing dead units, 199
      excessive in capacitors, 23               gear, 188                                    laptop display problems,
External (ext) trigger control,             FET (field-effect transistors)                      258–259
   oscilloscope, 89                                defined, 23                               overview of, 122–124
eye pattern, optical disc players,                 markings, 141
   247, 289                                        out-of-circuit testing on, 142     G
                                                   overview of, 140–141               gain
F                                                  substituting for, 218–219
                                            FET-VOMs, 23–24, 109–110
                                                                                            of amplifiers, 66
failures, component                                                                         defined, 63, 289
                                            filter capacitors, 62–63, 289                   substituting transistors,
      capacitors, 115–117
                                            flammability, of naphtha, 20–21                    219–220
      crystal clock oscillators, 120
                                            flash capacitor, digital camera danger,   gallium-arsenide diodes, replacing, 217
      crystals and resonators, 118
                                                183–184, 198                          game consoles, fixing disc players
      fuses, 123
                                            flat-panel displays, tips and tricks         incorporated into, 242
      inductor and transformer, 125
                                                   dangers within, 249                gate period, frequency counters,
      integrated circuits, 128
                                                   how they work, 248–249                107–109
      op amps, 130
                                                   how to fix, 250–251                GDS (gate, drain and source),
      potentiometers, 135
                                                   if it is worth fixing, 249            substituting transistors, 218
      relays, 137
                                                   what can go wrong, 249             germanium diodes, replacing, 217
      resistors, 132
                                            flatblade screwdrivers, 18–19             giga, 289
      switches, 139
                                            flip-up latches, 174–175, 231             glass small-signal diodes, choosing
      transistors, 142
                                            fluorescent tubes, dropped, 49               substitute, 217
      voltage regulators, 144
                                            FM (frequency modulation)                 glyptal, reconnecting screws with, 228
      zener diodes, 146
                                                   defined, 292                       GND input coupling setting,
failures, unit
                                                   example of silent shortwave, 56       oscilloscope, 83–84
      alive and awake but not quite
                                                   reading radio schematics, 164      graphics chip, laptop display
         kicking, 202–203
                                                                                         problems, 259
                                                                                                         Index            307
graticule, oscilloscopes                   heatsink grease                              IF (intermediate frequency) amplifiers
      defined, 77                                installing through-hole parts,               as common circuit, 298
      horizontal control settings, 91–92            221–222                                   defined, 67
      vertical control settings, 81,             workplace setup, 21                          radio schematics, 164
         83–84                             heatsinks                                    Illumination, oscilloscope screen
      viewing with illumination, 80              danger of, 198                            control, 80
grease, heatsink                                 dangers when repairing audio           images, fixing dust blobs in video
      installing through-hole parts,                amplifiers/receivers, 238              projectors, 283
         221–222                                 defined, 290                           impedance
      workspace setup, 21                        installing through-hole parts,               defined, 61, 290
ground loops                                        221–222                                   and load, 291
      defined, 289                               removing through-hole parts,                 vertical controls on oscilloscopes,
      preventing in workplace, 7                    212–213                                      82–83
ground symbols, schematics, 151            helical scan recording, VCRs and             inductance
guides, fixing VCRs and camcorders,           camcorders, 263                                 defined, 61, 290
   271–275                                 hemostats, workplace setup, 19                     DMMs testing, 8
                                           henries, specifying inductance, 290                impedance including effects
H                                          hertz (Hz), 60                                        of, 290
                                           heterodyning, intermediate                         of wire-wound resistors, 130–131
Hall-effect sensors, 289
                                              frequency, 298                            inductance meter
hamfests
                                           hex drivers, 18                                    out-of-circuit testing with,
      oscilloscopes at, 13
                                           HF Reject trigger control,                            125–126
      soldering irons at, 15
                                              oscilloscope, 90                                workplace setup, 29
      VOMs, FET-VOMs and VTVMs
                                           high-fidelity audio amplifier, 66, 293       inductive reactance, 61
          at, 24
                                           history, diagnosis using machine, 51–52      inductors
hand tools, workplace setup, 18–20
                                           Holdoff trigger control, oscilloscope,             call number, 152
hands, avoiding electric shock, 32
                                              90–91                                           defined, 61
hard drive players
                                           horizontal controls, oscilloscope,                 finding in battery-powered
      dangers within, 252
                                              78, 91–92                                          gear, 188
      disassembly of, 181
                                           Horizontal display settings,                       overview of, 124–126
      how they work, 251
                                              oscilloscope, 78, 93–95                         radio/TV, 191
      how to fix, 252–253
                                           Horizontal position, oscilloscope, 91        infant mortality, in product failure,
      if it is worth fixing, 252
                                           hot-melt glue gun, workplace setup, 26          44–45
      what can go wrong, 251–252
                                           house numbers                                inhalation
hard drives
                                                 avoiding transistors and chips               of contact cleaner spray, 110
      laptop problems, 255, 260
                                                    with, 27                                  of naphtha, 21
      MP3 player problems, 261–262
                                                 on transistors, 141                    initial evaluation, machine failure,
harmonically related waves, 290
                                           Howard W. Sams company, 149–150                 52–54
harmonics
                                           hum, fixing audio amplifier and              Input Coupling control, oscilloscope
      defined, 290
                                              receiver, 240                                   AC setting, 84–88
      sawtooth waves, 64
                                           humidity, workplace setup, 6                       DC setting, 84
      sine waves lacking, 63
                                           Hz (hertz), 61                                     defined, 78, 83
      square waves, 64
                                                                                              GND setting, 83–84
head crash, hard drives, 251
head drums, VCRs and camcorders            I                                            input points, finding on
                                                                                           schematics, 163
      cleaning, 269–270                    ICs (integrated circuits)
                                                                                        input sections
      fixing stuck tapes, 266–267                call numbers on, 152
                                                                                              diagnosing alive and awake but
      fixing tracking problems, 271              CMOS as, 286–287
                                                                                                 not quite kicking units, 203
      how they work, 263–264                     defined, 290
                                                                                              recognizing major features of,
      what can go wrong, 265                     discrete vs., 288
                                                                                                 191–192
head tips, cleaning VCRs and                     large-scale (LSI), 126–127,
                                                                                              working to output from, 205–206
  camcorders, 269–270                               224–225, 291
                                                                                        input stages
head-worn magnifiers, 19                         latching or latch-up in, 291
                                                                                              defined, 290
headphones, hearing damage from, 34              markings on, 128
                                                                                              push-pull audio amplifier
hearing damage, from repair work, 34             overview of, 126–128
                                                                                                 schematics, 161–162
heat-shrink tubing, 21, 38                       solder removal on, 102–103
                                                                                              shielding, 192
heat stress, product failure from, 47            in switching supplies, 159–160
                                                                                              shielding using mu-metal, 292
heaters, workplace setup, 6                      in switching supply failure, 160–161
                                                                                              of stereoless receiver, 55
308           Index
inside circuitry. See disassembly
installing new parts, 221–222
                                            L                                                 disassembly of laptops, 184
                                                                                              in flat-panels, 248
insulators, installing through-hole         lamps, video projector                            laptop display problems, 259
   parts, 221–222                                 dangers of, 277–278                         output circuitry for, 195
integrated circuits. See ICs (integrated          fixing problems, 279–280                    product failure from dropped, 49
   circuits)                                      how they work, 276–277                      toxicity of materials in, 249
integrated, defined, 290                          if it is worth fixing, 278                  video projectors. See video
intermediate frequency. See IF                    what can go wrong, 277–278                     projectors, tips and tricks
   (intermediate frequency) amplifiers      land, defined, 291                          lead dress, 228, 291
intermittents                               LAP (least accessible place),               lead-free solder
      after closing up case, 205               examining, 210                                 availability of, 16
      in LSIs, 225                          laptop computers                                  removing from through-hole
      thermal, 203–204                            dangers within, 256                            parts, 212
      vibration or position-sensitive,            disassembly of, 183–185                     using, 100–101
         204–205                                  finding voltage converters and        leaf switches, disc players and
Internet access, workplace setup, 22                  regulation, 188                      recorders
inversion, 290                                    fixing charging problems, 256–257           fixing door problems, 243
invert (inv) button, oscilloscopes, 111           fixing crashes, 256                         fixing playback problems, 245
inverted signal, 66                               fixing display problems, 258–259            mechanical wear of, 45
inverters                                         fixing drive problems, 260                  overview of, 137–139
      as common circuit, 298                      fixing other problems, 260–261              troubleshooting DVD player,
      flat-panel problems and, 249                how they work, 253                             57–58
      laptop display problems,                    if it is worth fixing, 255                  what can go wrong, 241
         258–259                                  physical stress damaging, 49–50       least accessible place (LAP),
      step-up versions of SMPS                    what can go wrong, 44, 253–255           examining, 210
         as, 299                                  working with broken, 230 -231         LEDs (light-emitting diodes)
inverting amplifiers                        large-scale integrated chips. See LSIs            in flat-panels, 249
      defined, 290                             (large-scale integrated chips)                 laptop failure, fixing charging
      reading schematics, 154–158           laser diodes, disc players and recorders             problems, 257
isolation transformer, workplace                  dangers of, 242–243                         repairing remote control without
   setup, 25                                      fixing laser problems in DVD                   diagram, 167
isopropyl alcohol, workplace setup, 20                players, 247                      legs, circuit, 62
                                                  fixing problems with, 246–248         lenses
J                                                 what can go wrong, 241–242
                                            laser optical head sled assembly, 197
                                                                                              disc players and recorders,
                                                                                                 242, 245
jacks                                       latches                                           video projector, 277
     laptop problems with, 185,                   reconnecting ribbon connectors        Level trigger setting, oscilloscope, 79, 90
        254, 257                                      with/without, 230                 LF Reject trigger control, oscilloscope, 90
     symbols for schematics,                      removing ribbon cables that           light-emitting diodes (LEDs), repairing
        151, 163                                      have, 174                            remote control without diagram, 167
jeweler’s screwdrivers, 18                        working with broken, 230–231          light tunnels, DLP projectors, 278, 282
jewelry                                     latching (or latch-up)                      lighting, workplace setup, 7
     avoiding electric shock, 31                  defined, 291                          lightning strikes, 47–48
     removing when servicing                layers                                      Line trigger control, oscilloscope, 89
        switchers, 234                            connection problems between, 46       line voltage
JFET (junction field effect transistors),         disassembly in, 175–177                     converting to DC in AC-powered
   140–141, 219                                   reassembling screws using, 231–232             products, 299
joined conductors, schematic                layout controls, oscilloscope, 78                 defined, 291
   symbols, 151                             LCDs (liquid crystal displays)                    measuring, 73
                                                  avoiding electric shock in            linear power supplies
K                                                     monitors and TVs, 33                    audio receivers using, 237
keyboards                                         of backlight inverters, 191                 defined, 191
      disassembling laptop, 184                   defined, 291                                switch mode power
      fixing laptop, 260–261                      digital oscilloscope, 10–11                    supplies vs., 299
      problems on laptops, 255                    disassembly of, 177–178                     in traditional AC-powered gear, 25
kilo, 290                                         disassembly of digital cameras, 183         troubleshooting AC-powered
Krazy Glue, in workplace, 26–27                   disassembly of DVD players,                    gear, 89
                                                      179–180                           linear region, of digital gates in ICs, 67
                                                                                                      Index            309
linear taper pots, 134                            transistor, 141                    multichannel encoding, audio
linear voltage regulators                         voltage regulator, 143–144           amplifiers and receivers, 236
      defined, 298                                zener diode, 145                   multiplexed, 292
      overview of, 143–144                  matrixed, 248, 291
      switch mode voltage regulator         mechanical wear, product failure         N
         vs., 299                             from, 45
                                                                                     N-channel FETs (field-effect
linearity, 66, 291                          mechanisms, recognizing major
                                                                                       transistors), 140–141
liquid crystal displays. See LCDs (liquid     features, 195–197
                                                                                     nano, 292
   crystal displays)                        mega, 291
                                                                                     naphtha, workspace setup, 20–21
liquids, damaging electronics, 50           menus, digital vs. analog scopes, 87
                                                                                     NC (no connection)
load, defined, 291                          mercury vapor arc lamps, and electric
                                                                                           defined, 292
loading problems, VCRs and                    shock, 33
                                                                                           relays, 135
   camcorders, 266–269                      merged conductors, schematic
                                                                                           switches, 138
log taper pots, resistance on, 134            symbols, 151
                                                                                     needlenose pliers, 19
logic analyzer, workplace setup, 29         metal cabinets, sorting components
                                                                                     negative charge, batteries, 60
low-frequency rolloff, 85–86                  in, 28
                                                                                     negative feedback, 162, 292
LSIs (large-scale integrated chips)         metal film type resistors, 131
                                                                                     NiMH (nickel metal hydride)
      defined, 291                          metal-oxide-semiconductor field-effect
                                                                                       rechargeable batteries, 105
      overview of, 126–127                    transistors. See MOSFET (metal-
                                                                                     NO (normally open) contacts,
      resoldering, 224–225                    oxide-semiconductor field-effect
                                                                                       135, 138
                                              transistors)
                                                                                     noise
M                                           metal oxide type resistors, 131
                                                                                           diagnosing comatose or crazy
                                            micro, 291
magic smoke                                                                                   units, 202
                                            microfarads, capacitors, 113–114
     defined, 291                                                                          on digital multimeters, 73
                                            microprocessors, diagnosing comatose
     in reassembly errors, 227                                                       nonintegrated-circuit stage. See discrete
                                              or crazy units, 202
     respecting maximum inter-                                                         (nonintegrated-circuit) stages
                                            milli, 291
        electrode voltages, 220                                                      Normal sweep mode control,
                                            mixer
magnets                                                                                oscilloscopes, 91
                                                  defined, 298
     interacting with electricity, 61                                                NP (non-polarized) electrolytic cap,
                                                  intermediate frequency, 298
     workplace setup, 26                                                               capacitors, 115
                                                  radio schematics, 164
magnifiers                                                                           NPN bipolar transistors, 140–141,
                                            mode switch, VCRs and camcorders,
     using stereo microscopes,                                                         218–219
                                              268–269
        24–25                                                                        nylon gears, disc players and
                                            modulation, 292
     workspace setup, 19                                                               recorders, 243
                                            moisture, hard drive failure, 251
manufacturers
                                            MOSFET (metal-oxide-semiconductor
     diagnosing machine with
                                              field-effect transistors)              O
        history, 51
                                                  causes of failure, 142             ocean air and water, corroding
     issuing ECOs for design errors,
                                                  laptop failure and, 254              electronics, 50
        48–49
                                                  overview of, 140–141               ohmmeters, 209–210
     no longer selling schematics or
                                                  substituting for, 219              ohms
        service manuals, 149–150
                                                  when it is worth fixing                 defined, 60
     obtaining components from, 223
                                                     laptops, 255                         in impedance. See impedance
     understanding design
                                            motherboards, laptops                         specifying resistance in.
        philosophies and quirks of, 41
                                                  charging problems, 257                     See resistance (ohms)
markings
                                                  display problems, 259              Ohm’s Law, 60, 292, 297
     capacitor, 113–115
                                                  failure from heat stress, 47       one hand rule, avoiding electric
     crystal and resonator, 118
                                                  overview of, 253–254                 shock, 32
     diode, 121
                                                  when it is worth fixing, 255       online sources
     fuse, 122–123
                                            Mouser Electronics, 223                       finding parts, 223
     inductors and transformers, 124
                                            moving parts in units, diagnosing, 203        Internet access near
     integrated circuit, 128
                                            MP3 players                                      workspace, 22
     op amp, 129
                                                  disassembly of, 180–181                 product disassembly instructions,
     potentiometer, 134
                                                  fixing hard drives in, 251–252             171, 183–184
     relay, 136
                                                  tips and tricks for, 261–262            service manuals and
     resistor, 131–132
                                            MP4 players, 261                                 schematics, 150
     switch, 138
                                            mu-metal, 292
310           Index
op amps (operational amplifier),                resistors, 133                            saving from dead machines, 214
   128–130                                      switches, 139                             scrap boards for, 28–29
open circuits or open, 62, 292                  transistors, 142                          stockpiling assortment of, 27–28
optical disc players, recognizing               voltage regulators, 144             passive elements, 292, 296
   mechanisms, 195                              zener diodes, 146                   PC-based scopes, 13
optical drives, laptop repairs, 255, 260   output cables, flat-panel dangers, 249   PCBs (printed circuit boards)
optical path, fixing video projectors,     output current, switcher shutdown due          bad connections in multilayer, 46
   279–280                                   to unsafe, 236                               defined, 293
opto-isolator, switching power             output points, finding on big                  soldering irons for, 14
   supplies, 160                             schematics, 163                        PDAs
oscillators, 67–68, 299                    output stages                                  disassembly of, 181
oscilloscope, vertical controls                 in audio amplifiers/receivers,            physical stress damaging, 49
      Alternate or Alt, 88                         237–239                          peak inverse voltage (PIV), 216–217, 293
      Bandwidth Limit, 87                       defined, 292                        period, waveform, 63
      Chop, 88                                  diagnosing alive and awake but      pet-safe workplaces, 5–6
      Input Coupling (AC/DC/GND),                  not quite kicking units, 203     phase, 65, 293
         83–87                                  diagnosing comatose or crazy        phase-locked loops. See PLLs (phase-
      layout of, 78                                units, 202                          locked loops)
      Probe Compensation, 81–82                 reading schematics, 154             phase noise, 68
      Settings, 78                              recognizing features of, 194–195    Phillips screwdrivers, 18–19
      Vertical Input Attenuator or              working to input from, 205–206      phones, physical stress damaging, 49
         Volts/div, 81–83                  output transistors, 38                   photodetectors
      Vertical Mode, 87–88                 overcurrent problems, audio                    defined, 293
      Vertical Position, 87, 256             amplifiers/receivers, 239                    repairing DVD players, 165
      X-Y, 88–89                           overheating                                    repairing remote control without
oscilloscopes                                   of diodes in power supplies, 35               a diagram, 167
      analog, 9–10, 12                          fixing video projectors, 281–282    Photofact manuals, 149–150
      basic function of, 9                      fuse protection on audio            photos
      buying, 13–14                                amplifiers, 123                        before disassembling devices, 22
      cursor controls, 96                       product failure from, 47                  before disassembling laptops,
      delayed sweep controls, 92–96             from too much current through                 183–185
      digital, 10–12, 96–98                        components, 47–48                      of layers during disassembly,
      electrical damage from, 34–36             voltage regulators, 38, 144                   176–177
      fixing VCRs and camcorders, 271           zener diodes, 146                   physical damage, to equipment you are
      as frequency counter, 23             overvoltage                                 fixing, 36–38
      horizontal controls, 78, 91–92            in crystal clock oscillators, 120   physical injury
      logic analyzer as offshoot of, 29         electrical damage from, 35                avoiding, 33–34
      overview of, 76–78                        product failure from, 47–49               from video projectors, 279
      PC-based, 13                                                                  physical stress, and product failure,
      screen controls, 78–81               P                                           49–50
      signal tracing and diagnosis. See                                             pico, 293
                                           P-channel FETs (field-effect
         signal tracing and diagnosis                                               picofarads, 106–107, 113–114
                                              transistors), 140–141
      spectrum analyzer as type of, 30                                              pictorial diagrams, 147, 149, 293
                                           p-mount cartridge, removing from
      trigger controls, 79, 89–91                                                   picture tubes, danger of TV, 177–178
                                              turntable, 178
      using sawtooth waves, 64–65                                                   piezoelectric effect, 117
                                           parallel circuit elements, 62
      vertical settings, 78                                                         pin basing, substituting transistors, 219
                                           parallel, defined, 292
out-of-circuit testing                                                              pinch roller, VCRs and camcorders,
                                           parts
      capacitors, 117                                                                  267–268
                                                 choosing substitute, 215–220
      crystal clock oscillators, 120                                                PIV (peak inverse voltage), 216–217, 293
                                                 diagnosing alive and awake but
      crystals and resonators, 118                                                  pixels
                                                    not quite kicking units, 203
      diodes, 122                                                                         defined, 293
                                                 ensuring product safety after
      fuses, 124                                                                          in flat-panels, 248–249
                                                    repair, 38
      inductors and transformers,                                                   plasma screens. See flat-panel displays,
                                                 finding, 223
         125–126                                                                       tips and tricks
                                                 installing new, 220–222
      integrated circuits, 128                                                      plastic capacitors, 115
                                                 removing through-hole parts,
      op amps, 130                                                                  plastic contamination, soldering
                                                    211–213
      potentiometers, 135                                                              irons, 99
                                                 saving damaged boards, 223–224
      relays, 137                                                                   plastic, in potentiometers, 134
                                                                                                    Index           311
plastic-melting iron, workplace                 choosing, 22                       Pull X10 control, oscilloscopes, 92
   setup, 15                                    connection, 103–105                push-pull audio amplifiers
playback, disc players and recorders,           current, 105                             dangers when repairing, 238
   245–246                                      current blasting with high-              defined, 293
PLLs (phase-locked loops)                          current, 209–210                      installing through-hole
      concept of, 68                            diagnosing alive and awake                  parts, 222
      as multistage circuit, 299                   but not quite kicking units,          malfunctions in, 162–163
      in older frequency                           202–203                               schematics, 161–162
         synthesizers, 298                      diagnosing comatose or crazy             substituting for, 219
plugs, product failure from bad, 45                units, 202                      PWM (pulse-width modulation)
plumbing applications, of acid-core             diagnosing dead units, 199–201           defined, 64, 293
   solder, 16                                   electrical damage to, 35                 fixing switchers, 235
PNP bipolar transistors, 140–141,               fixing audio amplifiers/                 switching regulators and, 143
   218–219                                         receivers, 238
polarity                                        fixing flat-panels, 250
      coaxial plugs for power                   fixing laptop computers,           Q
         supplies, 103                             254–255, 257                    quasi-complementary, 287
      defined, 61, 293                          fixing stuck tapes in VCRs and     Quickfact manuals, 149–150
      FET transistors, 140–141                     camcorders, 266                 quiescent current, 293
      markings on capacitors, 114–115           heat stress failure in, 47
      measuring DC voltage with                 large diodes in, 120
         DMMs, 72                               overview of, 299                   R
      NPN vs. PNP transistors, 140–141          reading schematics, 154            radio-frequency. See RF (radio-
      push-pull audio amplifier                 recognizing major features,           frequency)
         schematics, 161–162                       187–191                         Radio Shack, 223
      substitute electrolytics and, 216         voltage, 105                       radios
      substituting transistors and, 218   power surges, and product                       reading schematics, 164
polarized, defined, 293                      failure, 48                                  recognizing input features, 191
polarizers, LCD projectors, 278           power transistors, 293                          signal processing of frequency-
poles, relays, 135                        preamplifier or preamp                             synthesized, 193
poles, switches, 138                            CD and DVD player                         thermal intermittents in, 204
portable DVD players                               schematics, 165                 rails, power supply
      disassembly of, 179–180                   defined, 299                              defined, 293
      fixing. See DVD players and               repairing remote without                  diagnosing comatose or crazy
         recorders                                 diagram, 167                              units, 202
positive charge, in batteries, 60         precision                                       performing current blasting for
potentiometers (pot)                            digital multimeter, 8–9, 71–72               short across, 209
      call numbers, 152                         frequency counter, 108                    push-pull audio amplifiers, 161
      overview of, 133–135                      oscilloscopes not intended         rainbow effect, reducing in video
      transistors vs., 140                         for, 77                            projectors, 276–277
      using contact cleaner spray         primary lithium coin cell                RAM, and laptop computers,
         on, 110                             batteries, 201                           255–256
power                                     primary side dangers, switchers, 234     reactance
      disassembly of PDAs, 181            printed circuit boards. See PCBs                capacitive, 61, 290
      disconnecting before                   (printed circuit boards)                     defined, 61
         disassembly, 172                 Probe Compensation control,              real-time sampling, choosing digital
      disconnecting before removing          oscilloscope, 81–82                      oscilloscopes, 12
         boards, 211                      probes                                   reassembly. See reverse-order
      disconnecting when servicing              buying, 13                            reassembly
         switchers, 234                         causing electrical damage with,    receiver module, remote controls,
      dissipation of resistors, 131                34–35                              167–168
      dissipation when substituting       products                                 receivers, disassembly of, 177
         transistors, 220                       as art, 40–41                      rechargeable batteries, diagnosing dead
      as watts, 297                             brains of modern, 41                  units, 201
      workplace setup for electrical,           failure of. See troubleshooting,   rectifiers
         6–7                                       factors in product failure             choosing substitute, 216–217
power supplies                            protection circuits, audio amps and             diagnosing nonfunctional units by
      avoiding electric shock, 32            receivers, 237                                  stages, 206–207
312           Index
rectifiers (cont.)                           rework stations, desoldering with, 102             physical injury, 33–34
      diodes as, 120–122                     RF envelope, VCRs and camcorders,                  switch-mode power supplies, 234
      fixing switchers with blown, 235          271–274                                         VCRs and camcorders, 266
      stocking up on, 28                     RF (radio-frequency)                         salt water, corroding electronics, 50
rectify, 62, 294                                    defined, 294                          sample rate
reflow soldering, in SMTs, 225                      radio/TV, 191                               digital oscilloscopes and, 10–12
regulation systems, battery-powered                 reassembly in devices with, 229             low performance of PC-based
   gear, 188                                        recognizing output stage, 194–195              scopes, 13
relays                                              starting at input when working        sampling, 65, 294
      call numbers, 152                                on, 206                            Sams Technical Publishing, 150
      overview of, 135–137                   rheostats, 133                               saturated state, 67
remote controls                              ribbon cables and connectors                 sawtooth waves, 64–65, 294
      battery leakage in, 49                        diagnosing alive and awake but        schematics
      repairing without a diagram,                     not quite kicking units, 203             call numbers, 152–153
         166–168                                    disassembly of, 174–175                     defined, 294
reset circuits, in comatose or crazy                disassembly of laptops, 184–185             diagnosing nonfunctional units by
   units, 202                                       disassembly of video                           stages, 206
resistance (ohms)                                      projectors, 179                          good, not bad, and miserable, 153
      defined, 60, 294                              in disc players and recorders, 241,         mega maps, 163
      and load, 291                                    246–247                                  overview of, 147–150
      measuring with DMMs, 8, 73–76                 physical damage to, 36–37                   push-pull audio amplifier, 161–163
      product failure from bad solder               product failure from bad, 45                reading, getting started, 155–160
         joints, 46–47                              reassembly errors, 228                      reading, practicing, 164–166
      voltage and, 62                               reconnecting, 229–230                       reading, symbols for, 150–151
resistors                                           repairing in stationary                     switching power supply, 159–160
      adjusting to cover the real trouble,             applications, 224                        when major components fail,
         42–43                               ripple, in power supplies, 294                        160–161
      call numbers for, 152                  rise time, 64, 294                                 working without, 166–169
      choosing substitute, 217–218           RMS (root-mean-square) value, 73             SCR (silicon-controlled rectifier), 161
      diagnosing cause and effect, 207       RoHS (Restriction of Hazardous               scrap boards, workplace setup, 28–29
      electrical damage from, 35                Substances), Europe, 16, 294              screen controls, oscilloscope
      fixing in audio amplifiers/            rosin                                              digital vs. analog, 97
         receivers, 239                             removing after soldering, 101               settings, 78
      overheating of, 47–48                         in solder, 16                               viewing real signal, 79–81
      overview of, 130–133                          solder wick covered with, 16–17       screwdrivers, 18–19, 173–174
      potentiometers as variable,            Rotation, oscilloscope screen                screws
         133–135                                control, 80                                     finding hidden, 173
      repairing remote control without       rows, on flat-panel screens, 249                   inserting in layers, 231–232
         a diagram, 168–169                  RTV, securing wires with, 229                      reassembly errors, 227
      stocking up on, 27–28                  rubber bumpers, disassembly of DVD                 reconnecting, 228
resolution, digital oscilloscope                players, 179–180                                removing everything else first, 173
   limitations, 11                           rules, disassembly, 172–173                        removing layer by layer, 175–177
resonant circuits, 67, 294                                                                sealants, reconnecting screws with, 228
resonators, 117–118, 152                     S                                            secondary batteries, diagnosing dead
Restriction of Hazardous Substances                                                          units, 201
                                             S-video, TV, 192
   (RoHS), Europe, 16, 294                                                                secondary side, switchers
                                             safety during repairs
reverse-order reassembly                                                                        dangers within, 234
                                                   audio amplifiers and
      common errors, 227–228                                                                    fixing caps on, 235
                                                      receivers, 238
      completing, 232                                                                     sections, reading schematic, 154
                                                   disc players and recorders,
      getting started, 228–229                                                            selectivity, receivers, 67, 294
                                                      242–243
      layers and cups, 231                                                                semiconductors
                                                   electric shock, 31–33
      reconnecting ribbons, 229–230                                                             component cooler spray for, 27
                                                   electrical damage, 34–36
      repairing broken latches, 230–231                                                         defined, 295
                                                   ensuring product after repair, 38
      screws, 232                                                                               failure from electrical stress, 47
                                                   flat-panel displays, 249
      snaps, 231–232                                                                            failure from overheating, 47
                                                   laptop computers, 256
reverse voltage, substituting/replacing                                                         failure from reversed polarity
                                                   overview of, 31
   diodes, 216–217                                                                                 of, 48
                                                   physical damage, 36–38
reversed polarity, 35, 48                                                                       failure in switchers, 234
                                                                                                       Index           313
       infant mortality of, 44–45           sine waves, 63–64, 73, 295                      installing surface-mount
       measuring resistance with            single-pole, double-throw (SPDT) leaf,             components, 222
          DMMs, 74                             disc players and recorders, 243              installing through-hole parts, 221
       shorting in failure, 62              Single sweep mode control,                      removing from through-hole
series, 61–62, 295                             oscilloscope, 91                                parts, 211–212
series pass transistor, 68, 143             sink, 295                                       removing surface-mount
service manuals, replacing schematics,      size, choosing substitute                          components, 213–214
    148–149                                    electrolytics, 215                           resoldering LSIs, 224–225
servos                                      sled motor, CD and DVD players                  saving damaged boards, 223–224
       of CDs and DVDs, 164–165                    fixing laser problems in DVD             workplace setup, 15–16
       as common circuit, 299                          players, 247–248               solder joints
       concept of, 68                              fixing playback problems, 245            desoldering, 101–102
       fixing laser problems in DVD                schematics, 165                          fixing laptop computers, 256
          players, 248                             what can go wrong, 241                   MP3 player problems, fixing,
       fixing VCRs and camcorders,          slide latches, removing ribbon cables              261, 262
          275–276                              with, 174                                    product failure from bad, 44–47
       using sawtooth waves, 64             Slope trigger control, oscilloscope, 90         technique for proper, 98–99
Settings control, oscilloscope, 78          sloppiness, in repair work, 42                  thermal, 204
Sharpie marker                              slow-blow fuses, 122                      solder suckers
       pulling wire connectors, 175         smart batteries, modern laptops, 254            desoldering with, 17
       removing ribbon cables, 174          SMD (surface-mount device)                      removing through-hole parts
shields, 295                                       defined, 295                                using, 213
shim tools, 180                                    workplace setup, 29–30                   spring-loaded, 17
shoes, avoiding electric shock, 32          SMPS (switch-mode power supplies)         solder wick
short circuit (short)                              avoiding electric shock                  desoldering tools, 16–18
       defined, 62, 295                                from, 32                             desoldering with, 16–17, 101–102
       fixing output stage in audio                as common circuit, 299                   diagnosing cause and effect, 208
          amplifiers/receivers, 238–239            dangers within, 198, 234                 installing new surface-mount
shotgunning, when all else fails,                  defined, 24, 295                            components, 222
    208–209                                        diagnosing dead units, 200               removing surface-mount
shunt, 295                                         failure of major components in,             components, 213–214
signal                                                 160–161                              removing through-hole parts,
       defined, 295                                how they work, 233                          211–213
       from transducers, 192                       how to fix, 235–236                soldering irons
signal generator, 23, 107                          if it is worth fixing, 234               ensuring product safety after
signal processing, 63–65, 192–194                  reading schematics, 159–160                 repair, 38
signal tracing and diagnosis                       transformers, 189–190                    melting plastic with, 15
       alive and awake but not quite               what can go wrong, 234                   physical damage from, 36
          kicking, 202–203                  SMT (surface-mount technology)                  shotgunning when all else fails,
       checking, 207–208                       components                                      208–209
       comatose or crazy, 202                      defined, 295                             solder, 15–16
       concept of circuit stage,                   installing, 222                          technique for using, 98–101
          206–207                                  removing on circuit boards, 214          workplace setup, 14–15
       dead, 199–201                               workplace setup, 29–30             solvent, naphtha used as, 20–21
       desperate measures when all else     snaps                                     source, 296
          fails, 208–210                           reconnecting, 231–232              Source trigger control, oscilloscope,
       to and fro, 205–206                         removal from DVD players, 180         89–90
       overview of, 199                            removal from laptop                Source trigger setting, oscilloscope, 79
       sometimes yes, sometimes no,                    keyboard, 184                  SPDT (single-pole, double-throw) leaf,
          203–205                                  removal from laptop LCD               disc players and recorders, 243
signals                                                housing, 185                   speaker magnet, cell phone
       fixing in audio amplifiers/                 separating, 173–174                   disassembly, 181
          receivers, 239–240                SNR or S/N (signal-to-noise)              speaker symbol, schematics, 151, 163
       hard drive failure and, 252             ratio, 295                             speakers
silicon-controlled rectifier (SCR), 161     sockets, product failure from bad               call number, 152
silicone sealer, securing wires with, 229      connections, 45                              recognizing output stage features,
SIM (subscriber identity module) card,      solder                                             194–195
    cell phone disassembly, 181                    desoldering tools, 16–18           spectrum, 296
314           Index
spectrum analyzer, workplace setup, 30      superheterodyne receivers, 298                 switches, 138
speed, substituting/replacing diodes or     surface-mount components, removing/            transistors, 141
   rectifiers, 216–217                         replacing on circuit boards,                voltage regulators, 143
spindles                                       213–214                                     zener diodes, 145
       fixing disc players and recorders,   surface-mount device (SMD)                symptoms, mistakes beginners make,
          242, 244–246                            defined, 295                           42–43
       fixing VCRs and camcorders,                workplace setup, 29–30              sync, 296
          268, 270                          surface-mount electrolytics, on           systems, machines as, 39
spring-loaders suckers, desoldering            polarized capacitors, 115
   with, 17, 102                            surface-mount resistors, markings
square waves, 63–64, 296                       on, 132                                T
stages                                      surface-mount technology. See SMT         tantalum capacitors
       defined, 63, 296                        (surface-mount technology)                   defined, 115
       diagnosing cause and effect,            components                                   failure of, 116
          207–208                           swabs, workspace setup, 19–20                   markings on, 114
       diagnosing nonfunctional units,      sweep control, oscilloscope, 91                 replacing, 215–216
          206–207                           sweep horizontal setting,                 tape-based camcorders, disassembly of,
stages, reading schematics                     oscilloscope, 78                          181–182
       CD and DVD players, 164              sweep rate, digital oscilloscope, 10–11   tape path, cleaning VCRs and
       inverting amplifier example,         sweep rate or time/div horizontal            camcorders, 269
          154–158                              setting, oscillos